Floods are the most frequent natural disasters in New Zealand – between 1968 and 2017, the country experienced more than 80 damaging floods. The Insurance Council of New Zealand calculated that payouts on claims for flood damage between 1976 and 2004 averaged $17 million per year in 2004 dollars. But this was just part of the total cost – for example, government expenditure on civil defence responses during flood emergencies alone averaged about $15 million per year over the same period.
Floods have cost an uncounted number of lives in New Zealand. Māori history tells of a pre-European flood in the Tūtaekurī area of Hawke’s Bay in which a party of 50 men, women and children were drowned when two streams rose. The early European settlers failed to realise the intensity of rainfall in New Zealand and how rapidly rivers could rise. The South Island’s broad gravel-bed rivers were particularly deceptive: they were usually shallow enough to wade across, but when in flood their currents were powerful. By 1870, just three decades after European settlers began arriving in large numbers, rivers had been responsible for 1,115 recorded drownings. Drowning became known as ‘the New Zealand death’.
Māori legend includes a story of a great flood. Tāwhaki, god of thunder and lightning, was almost murdered by his brothers-in-law. When he had recovered, Tāwhaki took his warriors and their families and built a fortified village on top of a mountain. Then he called to his ancestors – the gods – for revenge, and they let the floods of heaven descend. The earth was overwhelmed by the waters and the entire population perished. This was known as Te hurihanga i Mataaho (the overwhelming of Mataaho – one of the places that were destroyed).
Most floods occur when water from intense or persistent rain, or sometimes from melting snow, enters rivers, streams and lakes, causing them to overflow. High sea levels at river mouths may also increase flood levels. As climate change began to cause sea level rise, coastal settlements also became increasingly vulnerable to floods caused by high tides and storms.
In New Zealand, the heaviest rain commonly accompanies extra-tropical cyclones, depressions and frontal systems. Extra-tropical cyclones, such as the storm of 28 January 1936 (considered New Zealand’s worst storm in the 20th century) and Cyclone Bola in 1988, have caused major floods, but such storms reach New Zealand relatively infrequently.
Much more common is flooding caused by the heavy rain that may accompany mid-latitude depressions and frontal systems moving over the country. New Zealand’s rugged topography often enhances the effects of these weather systems, as moist air forced up and over mountainous terrain condenses to produce additional precipitation. As a result, the western side of the Southern Alps is one of the wettest places in the world, with over 13 metres of rainfall per year in some places.
Floods may also occur after landslides triggered by severe weather or earthquakes dam rivers and streams. Water backs up upstream of the barrier and may cause a flood downstream if the dam gives way. About 200 dams were formed by landslides caused by the 2016 Kaikōura earthquake.
In the South Island, many major eastern rivers originate in the Southern Alps. Weather conditions in their mountain headwaters often produce floods in their lower reaches. The greatest flood ever observed on the Clutha River Mata-Au, New Zealand’s largest river in catchment area and volume of flow, occurred in 1878. It was the result of a succession of weather systems bringing in warm wind and rain, which melted the winter snow cover. At the height of this flood, more than 5,700 cubic metres of water poured down the lower reaches of the river every second.
Brief but intense rainstorms may produce flash floods – brief, powerful flows that can move even large boulders. A destructive flash flood occurred near Roxburgh in November 1992 after 80 millimetres of rain fell in just 45 minutes.
If large amounts of sediment enter the flood waters, for example from landslides, the flood may turn into a debris flow – a fluid mix of water, rock particles and vegetation, which may be as thick as wet concrete. In 2005, over 300 mm of rain fell in 24 hours near the township of Matatā in Bay of Plenty, triggering debris flows that heavily damaged more than 100 homes. Flash floods and debris flows also regularly damage bridges, resulting in expensive road maintenance in mountainous regions.
Most major rivers in New Zealand have a history of destructive floods, and European settlers soon discovered that floods were a recurring menace. Wellington’s first shiploads of settlers arrived at Petone in January 1840, intending to farm the broad Hutt Valley. Less than two months after they arrived, the Hutt River overflowed, inundating their huts and tents. After several more floods during the next few months, most of the settlers abandoned the Hutt Valley and moved to Thorndon, now part of central Wellington.
In Christchurch, settlers unwittingly chose a site for the city that was part of the constantly shifting channels of the Waimakariri River.
Blenheim was dubbed ‘Beaver Town’ or ‘The Beaver’ by early surveyors – like a beaver colony built in midstream, it was often completely swamped by the meandering Ōpawa River. The river also acted as an overflow channel for flood water from the Wairau River. During one period in 1893, the local newspaper, the Daily Times, noted:
Again the town has been inundated with Opawa back-water, and this time to a greater height than has been the case during winter. We have now had nine floods in 11 weeks, each of them being of sufficient size to entirely suspend business operations for two days. 1
In 1863, prospectors swarmed across Central Otago, camping along the streams and rivers where there was gold. The winter that year was severe, with thick snow blanketing the mountains. In July, warm rain deluged the region for six days, combining with melted snow to boost the rivers.
Between 25 and 27 July, rivers swelled to disastrous levels. During a single night the tributaries of the Molyneux (Clutha Mata-Au) – the Shotover, Kawarau and Arrow rivers – rose by 6 to 10 metres. The floods overwhelmed dozens of miners asleep in tents and makeshift huts on the river banks beside their claims, or even on terraces well above normal river levels. Eight huts disappeared from a beach in the Arrow Gorge, and by morning, only one tent of many was left on a terrace opposite Arrowtown. In the upper Shotover the torrent undermined a terrace, and a hut in which 15 men were living collapsed into the river; 12 of them drowned.
The sodden mountains gave way in landslides that sometimes blocked the valley floors. Weeks earlier, a landslide had dammed a stream running into the Shotover River at Sand Hills near Māori Point. Swollen by heavy rain, the stream burst through the barrier on 26 July and overwhelmed a camp, killing another 12 miners.
On the Arrow River, a landslide dam broke, releasing a wall of water that swirled through the lower areas of Arrowtown, sweeping away buildings and burying everything beneath several metres of gravel. Most miners escaped to high ground, but many lost all their belongings and equipment. By the end of August, more than 100 lives had been lost because of the Central Otago floods.
Although the Central Otago floods of 1863 resulted in more deaths, the Clutha River floods of September and October 1878 caused the worst damage. Heavy rain and melting snow in the headwaters of Lake Wānaka produced floods that lasted for three weeks, inundating settlements and demolishing dozens of bridges along the whole length of the river. A 1938 account described the Clutha in flood:
[i]ts angry surface [was] strewed with dead horses and cattle, houses, bridges, furniture, timber and farmstacks. Some days the spring sun shone with a ghastly pleasantry on the devastated towns, while 100 miles away more heavy rain on the mountains was preparing still greater strength for the flood. 2
Twenty-one people were killed in the Kōpuawhara flood of 1938 – the largest number of fatalities from a 20th-century New Zealand flood. It is a sobering reminder of the dangers of building on low-lying land close to rivers.
In 1938, workers building the Wairoa to Gisborne railway near Māhia lived in huts and tents in public works camps along the banks of the Kōpuawhara Stream. No. 4 camp was in a river valley, but appeared to be safely above the level of the tiny stream. In the early hours of 19 February, a cloudburst caused a flood that sent a wall of water nearly 5 metres high down the stream.
About 3.30 a.m., water began to pour across the lower levels of No. 4 camp. A worker raised the alarm, banging the cookhouse gong and beating on hut doors; his body was later found 5 kilometres downstream. Men struggled to get to higher ground through the rising water, and many scrambled onto the roofs of huts. Most of the huts collapsed and the people on them were swept away. Two men died after wading into the torrent to try to find the camp’s waitress; her hut had been one of the first carried away.
Eleven men at No. 4 camp climbed onto a truck used for carrying shingle to escape the flood. The truck was toppled over and the men were washed away. Only the bonnet was found later, 10 kilometres downstream.
Fourteen people survived by climbing onto the cookhouse roof, then leaping to the roof of the adjoining caterer’s quarters as the cookhouse collapsed. An elderly man tied himself to a hut with electric cable, and held a five-year-old girl above the water for an hour. Eventually a rope was carried from higher ground to the rooftop survivors, and they were hauled to safety.
At No. 2 camp, about 5 kilometres downstream, 47 people were sleeping. Men woke to find waves dashing against their tents, but the alarm was raised in time for everyone to struggle through the rising water to high ground.
In all, 20 men and one woman at No. 4 camp were drowned. One man was drowned at Boyd's Camp. at the Gisborne end of the railway line. He was swept away by the Maraetaha Stream, which was in flood as a result of a downpour produced by the same weather system. In 1942 a memorial which named all 22 victims was set up to mark the site of No. 4 camp.
In late January 1984, devastating floods struck Invercargill and other parts of Southland. The cause was a combination of north-westerly rain in the mountain headwaters of Southland rivers, and a slow-moving southerly front that dumped heavy rain over the lowlands. Steady rain on 26 January – a record one-day total of 84.8 millimetres – had by 9 p.m. caused extensive surface flooding in the streets of Invercargill, Riverton, Ōtautau and Bluff. Waterways soon overflowed, and by 4 a.m. a state of emergency had been declared in Invercargill. By morning many streets, houses, shops and factories were under water, and streams were sending torrents of water through Invercargill. Levels rose still further as high tide prevented flood water draining into the Invercargill estuary. By mid-morning of 27 January, the state of emergency included all of Southland. The rain had stopped by noon, but the rivers continued to rise.
Army and air force personnel were dispatched south to aid rescue efforts. An estimated 5,000 people abandoned their homes, and helicopters lifted dozens of people from rooftops. Roads and railway lines were submerged, cutting off the region from the rest of New Zealand. Water up to 3 metres deep flooded Invercargill airport, partly submerging 10 light aeroplanes.
It would be a week before most people could return to their homes, and several weeks before the state of emergency was lifted. When the flood receded, extensive damage was revealed: at least 1,200 homes were uninhabitable, and residents eventually discarded more than 5,000 tonnes of ruined personal possessions. Hundreds of flooded cars had been abandoned. No human lives were lost, but livestock losses were heavy – more than 12,000 sheep, 330 pigs, 100 cattle and 75 deer were drowned. A relief appeal raised more than $3 million, while tens of millions of dollars were paid out in insurance claims.
One of the most damaging cyclones to have hit New Zealand in the last two centuries is Cyclone Bola, which struck the Hawke’s Bay and Gisborne–East Cape region in March 1988. The former tropical cyclone slowed as it moved over the area, resulting in over three days of torrential rain. Worst affected was the hill country inland from Gisborne, where strong winds forced warm moist air up and over the hills, augmenting the rainfall from the storm itself. In some places, over 900 millimetres of rain fell in 72 hours, and one locality had 514 millimetres in 24 hours.
Flooding from Cyclone Bola had a devastating effect on horticulture in the East Coast region. The damaged crops included 3,000 tonnes of grapes, 1,300 tonnes of squash, 7,000 tonnes of sweetcorn, 13,500 tonnes of tomatoes, and several million dollars in vegetables for the local market. Large amounts of horticultural produce were swept into the sea – fishing boats were dredging fruit from the sea floor in their nets for several months after the storm.
The ensuing floods overwhelmed river stopbanks, damaged houses, swept away bridges and sections of roads and railway lines, and destroyed parts of Gisborne’s main water-supply pipeline. Three people died in a car that was swept away by flood waters, and thousands were evacuated from their homes – 3,000 in Gisborne, 300 in Wairoa, and 400 at Te Karaka.
The downpours triggered innumerable landslides on the region’s hillside pastures. Some farmers lost 30% of their grazing area, with landslide scars taking decades to heal. Huge quantities of sediment were dumped into the flood waters. One 11,000-hectare catchment deposited a million tonnes of sediment into the Waipāoa River. Thick sediment from the ebbing floods smothered pastures, orchards, and crops ready for harvest. Cyclone Bola was estimated to have cost $90 million in losses to horticulture and farming, and the cost to the taxpayer was around $112 million.
Between 14 and 17 February 2004, intense rain (up to 300 millimetres in two days) fell on land already saturated after previous severe weather. Worst affected were rivers in South Taranaki, Manawatū–Whanganui, Wellington and Marlborough. The Manawatū River peaked at its second-highest level on record. Many rivers breached their banks, spilling silt-laden flood water through towns and across farmland. A number of rural communities were evacuated. At the height of the emergency, about 2,300 people had to leave their homes and farms.
About half the roads in the Manawatū–Whanganui region were closed and more than 20 bridges were damaged. The civil defence operation was the largest in 20 years, with air force helicopters rescuing stranded people and dropping supplies to communities cut off by flood waters.
In Marlborough, the Waitohi River flooded parts of Picton, and 500 people were evacuated for fear an overflowing water supply dam might collapse.
The February floods cost over $112 million in insurance payouts, and the government granted $135 million in aid to farmers. Around 2,600 farmers were affected by the flooding, with some having to abandon their properties. The total economic impact was about $400 million.
In July 2004, eastern Bay of Plenty was extensively flooded when a frontal system stalled over the area, causing prolonged and intense rain. The Whakatāne River spilled into the town’s central business district and the suburb of Awatapu. Water had to be released into the Rangitāiki River from the Matahina Dam to prevent the dam bursting. The Rangitāiki overflowed above Edgecumbe, creating a 100-metre-wide breach in its stopbanks. Flood waters entered Te Teko and Edgecumbe, and swamped some 17,000 hectares of farmland.
At the height of the floods, about 3,200 people had to leave their homes, many sheltering in evacuation centres and on local marae. A swarm of shallow earthquakes north-east of Rotorua added to the misery, triggering many landslides on saturated hillsides. More than 450 farms were affected by the floods, with over 200 homes made uninhabitable.
In April 2017, remnants of two tropical cyclones, Debbie and Cook, passed over New Zealand within a week of each other. Cyclone Debbie merged with a cold front to form a deep depression which dumped up to three times the normal April rainfall in three days on some areas. The Rangitāiki River again rose rapidly, and on 6 April a section of stopbank at Edgecumbe gave way. A torrent of water flooded into the town and inhabitants had just a few minutes to flee their homes. Many were marooned and had to be rescued by boats, tractors and trailers. About 2,000 people were evacuated from Edgecumbe.
In the South Island, Kaikōura was cut off when State Highway 1 south of the town was blocked by mudslides washed down from loose landslide debris from the November 2016 Kaikōura earthquake.
Rain from the remnants of Cyclone Cook caused flooding, slips and power outages along the east coast of the North Island. Hard-hit areas included Thames-Coromandel, Matamata, Tauranga, Whakatāne and parts of Hawke’s Bay.
On Friday 27 January 2023, the beginning of a long weekend marking the region’s anniversary, Auckland city was hit by an unprecedented deluge, with 249 millimetres of rain recorded at the airport in 24 hours (the previous record was 161.8 millimetres). Stormwater infrastructure was soon overwhelmed and low-lying areas were flooded. Hundreds of landslides fell from cliffs of relatively soft sandstone and mudstone rock.
Three people died in Auckland and one at Onewhero in Waikato. Several hundred people were injured. Hundreds of houses were rendered uninhabitable and thousands of cars had to be written off because of water damage. Nearly 100 roads were closed because of flooding or slips. There was also flooding in Northland, Waikato and Bay of Plenty. Both State Highway 1 to Whangārei and the North Auckland railway line were impassable for months. Power outages and loss of cellphone coverage was widespread.
Just a fortnight after the Anniversary weekend storm, a much wider area – the entire east coast of the North Island, from Northland to Wairarapa – was struck by the remnants of Cyclone Gabrielle. As with Cyclone Bola, Gabrielle’s movement was slowed by a blocking anticyclone, prolonging the heavy rain. In some areas, 300 to 400 millimetres fell in 48 to 60 hours. Esk Valley, just north of Napier, received 500 millimetres in 24 hours, half of this in six hours in the middle of the night.
Eleven people died (eight of them in Hawke’s Bay) and dozens were rescued from rooftops by helicopters and the brave skippers of small boats. Once again, hundreds of people were injured, hundreds of homes became uninhabitable, and there were widespread power outages and loss of cellphone coverage. An estimated 10,000 people were forced to leave their homes, at least temporarily.
The total cost of these events was expected to exceed $10 billion, although some of this would be covered by insurance. By way of comparison, the Canterbury earthquakes of 2010/11 are thought to have cost the economy around $40 billion.
Early European settlers in New Zealand commonly chose to live next to rivers or lakes, as these were sources of fresh water and the adjacent plains usually had fertile soil. Coastal areas were also attractive sites. Consequently, about two-thirds of New Zealanders now live in areas that are naturally prone to flooding. Nearly 70% of towns and cities with populations of over 20,000 have river flood problems.
Human activity has also increased the likelihood of flooding. Large areas of native forest were cleared by both Māori and Europeans, leading to more rapid run-off of rain into stream networks, and to erosion that raised the levels of river beds.
In urban areas, buildings, footpaths and roads have replaced ground that would normally soak up falling rain. This leads to surface flooding during heavy rain, and increases the run-off into storm drains, causing higher water levels in local streams.
At Avonhead lived one Mister Bray,
Who every morning used to say
‘I should not be much surprised today
If Christchurch city were swept away
By the rushing, crushing, flushing, gushing Waimakariri River.’
This 1860 poem by Canterbury politician and poet Crosbie Ward refers to the engineer William Bray, who predicted that the Waimakariri River would break its banks and run through Christchurch. His prediction came true in February 1868, when the Waimakariri overflowed into the Avon River and other old stream channels, flooding parts of the city.
Early attempts at controlling rivers were piecemeal – to protect a homestead, individuals would build stopbanks that sometimes deflected river flow onto neighbouring properties. The River Boards Act 1884 set up a national network of river management. But a single river was often controlled by severaldifferent boards.
In April 1938, a flood in Hawke’s Bay left the lower Esk Valley buried under metres of silt. The disaster spurred an effort to control floods, resulting in the Soil Conservation and Rivers Control Act 1941. This pioneering legislation, as well as the later Water and Soil Conservation Act of 1967, authorised catchment boards to deal with complete river systems – both in carrying out river control works and in controlling erosion in their catchments.
In 1991, the Resource Management Act turned over flood control to regional and territorial authorities as part of their obligation to avoid or mitigate natural hazards.
A number of techniques are used in New Zealand to control rivers. Thousands of kilometres of stopbanks have been built to keep high river levels within the channels; this has allowed dense settlement of areas such as Christchurch, Hutt Valley and the Heretaunga Plains. Christchurch, for example, is situated mainly on a flood plain. It occupies part of the huge alluvial fan of the Waimakariri River, which has frequently changed course, sometimes entering the sea as far south as Lake Ellesmere. Stopbanks now keep the Waimakariri in one place along the northern edge of the fan.
Stopbanks can be very cost-effective. It has been estimated that spending $10 million on a stopbank on the lower Buller River would have prevented $100 million of damage caused when the river inundated the south-eastern part of Westport in July 2021.
Stopbanks, however, may also cause problems. Near Franz Josef, stopbanks built to protect the township narrowed the bed of the Waiho River. The riverbed built up so much that in 2011 the State Highway 6 bridge had to be raised to stay clear of the river. In 2016, the river breached the stopbank, flooding two hotels and a campground. The Waiho bridge was washed away in a flood in March 2019.
In 1877, Hawke’s Bay County began to plant willows along river margins to help prevent bank erosion. Less than 20 years later, in 1894, the county set up a sub-committee to deal with ‘the willow nuisance’. The trees, which impeded the flow of rivers during floods, continued to be a worry for the next half-century.
One method of preventing rivers overflowing their stopbanks is to lower the river bed by removing gravel. Speeding the passage of flood waters through an area can involve straightening river channels and removing obstructions such as vegetation. Another method of flood control is damming or diverting flood water until river levels drop. To protect communities downstream, some river stopbanks have floodgates that can be opened to deliberately spill flood water onto less densely settled farmland. The Moutoa Sluice Gates, for example, divert excess water from the Manawatu River into a floodway that rejoins the river 10 kilometres downstream.
New Zealand’s large dams were built for power generation, irrigation and water supply, but they have at times buffered major floods.
A common expression in discussions about floods is that a particular event is a ‘one-in-100-year flood’. Some understand this to mean that this event could only happen once every 100 years, which may give them a false sense of security. However, what the phrase really means is that there is a 1% chance of it occurring in a given year, and if it did happen, the probability of it happening again the following year (or in any subsequent year) would still be 1%. As climate change continues to affect New Zealand, ‘one-in-100-year’ events are likely to increase in frequency and intensity.
One method of preventing rivers overflowing their stopbanks is to lower the riverbed by removing gravel. Speeding the passage of flood waters through an area can involve straightening river channels and removing obstructions such as vegetation. Another method of flood control is damming or diverting flood water until river levels drop. To protect communities downstream, some river stopbanks have floodgates that can be opened to deliberately spill flood water onto less densely settled farmland. The Moutoa Sluice Gates, for example, divert excess water from the Manawatū River into a floodway that rejoins the river 10 kilometres downstream.
New Zealand’s large dams were built for power generation, irrigation and water supply, but they have at times buffered major floods.
While heavy engineering work such as stopbanks remain important, other solutions have been implemented. Councils can give rivers and streams in urban landscapes more space to take their natural course, be more vigilant about keeping rivers, streams and stormwater drains clear, and build and maintain wetland areas, which can better absorb rainfall.
Regional and district councils have the primary responsibility for managing flood hazards. They monitor rainfall, river flows and lake levels, and maintain flood protection works.
The earliest indications of potential flooding are the heavy rain warnings issued by weather forecasters. In addition, councils independently operate networks of automated instruments that measure rainfall and river levels. Data from these instruments, and high rainfall rates or rising river levels, may trigger automatic warnings to staff. During extreme events such as the February 2023 Esk Valley flood, automated instruments may stop transmitting or be destroyed.
Council staff also use computer models of rainfall and river flow to determine likely rises in river and lake levels downstream, and supply warning information to communities. Among other operations, they organise evacuations, build sandbag barriers, and close roads. Most councils maintain websites and telephone services that inform the public about rainfall and river levels. These too may be vulnerable to disruption during extreme events.
Information on lake and river water levels, river flows and sediment loads also goes into a national Water Resources Database, managed by the National Institute of Water and Atmospheric Research (NIWA).
In collaboration with regional councils, NIWA has developed techniques for forecasting floods using computer models of atmospheric conditions and river catchments. These enable councils to build flood resilience into their planning, and allow people to see where floods are more likely to occur when they are considering purchasing a home.
Frederick W. Furkert, a New Zealand public works engineer in the 1920s, was experienced in river problems. He recalled a statement that the first engineer he worked under used to make: ‘You have never seen it rain so hard that you could not imagine it raining a little harder or a little longer; only one of those conditions is necessary to make a bigger flood than you have ever seen.’ 1
NIWA also carries out research on climate change and long-term climate cycles, such as the El Niño–Southern Oscillation that affects New Zealand’s weather patterns. In El Niño years, stronger westerly winds bring more rain to southern and western regions. By contrast, the La Niña pattern brings rainy conditions to the north-east of the North Island.
NIWA climate modelling has found that climate change is likely to increase both the frequency and the intensity of extreme rainfall events. The changes will be different in different parts of the country. Floods may also be made worse by changes to other parts of the environment, such as increased amounts of sediment in rivers and changing shapes of river beds. Also, as temperatures rise, water falling on mountains such as the Southern Alps is less likely to turn to snow. This may mean rivers will flow at higher levels in winter rather than remaining low until the spring snowmelt.
As even major river works may not prevent flooding, measures to alter the way flood-prone areas are developed were put in place. Under the Resource Management Act 1991, territorial and regional authorities can regulate land use and construction on high-risk flood plains. Local authorities use instrument records and historical accounts of rainfall, river levels and floods to determine hazard zones, based on the probability of land being inundated. Members of the public can obtain a Land Information Memorandum (LIM) on any property from a councils; this includes information on the risk of the property being flooded.
In many regions, riverside zones in urban areas are reserved as parks, sports fields and parking areas. This allows flood water to pond where it will cause minimal damage. In some areas, developers are required to allow overflow spaces in new subdivisions which will allow flood water to peak and recede without causing damage. New buildings are also often built with stormwater detention tanks which store water during intense rainfall events and then slowly release it, easing the pressure on rivers that are already full.
Central government and councils are also under pressure to help some communities retreat from areas impacted by floods, sea-level rise and coastal flooding. This will allow people to move to safer areas, and give more room for flood waters. They can do this by purchasing at-risk properties where the risk to life is high and it is not feasible to make them safe. After Cyclone Gabrielle in 2023, many hundreds of residential properties in Auckland were deemed to be in this category. This strategy is expected to become a costly burden on the authorities in the future. It is a particularly difficult issue for Māori communities, who have generations-long connections to their (often low-lying) land, and may have nowhere else to go as a result of significant land loss in the past.
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Mosley, M. Paul, and Charles P. Pearson, eds. Floods and droughts: the New Zealand experience. Wellington: New Zealand Hydrological Society, 1997.