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Water quality

by  Mike Scarsbrook and Kit Rutherford

Clean, fresh, clear water is the essence of life, and New Zealand has it in bucket loads – for drinking, washing, swimming, fishing, boating, irrigating and hydroelectricity. To control its use and protect its quality requires a commitment from government, industry, farmers and communities.

Water quality in New Zealand

By world standards New Zealand has high-quality water available for many uses. A plentiful supply is something that New Zealanders have taken for granted since the days of early settlement. Maintaining the quality has, for a long time, been largely due to the low population and the low level of contaminants from farming and urban development.

How we use water

People have contact with water in many ways, including:

  • wading in streams and rivers while tramping or fishing
  • boating, swimming or fishing in lakes
  • washing or drinking from a reservoir or water tank.

Other uses include:

  • farm irrigation and drinking water for animals
  • effluent disposal
  • generation of hydroelectricity
  • as a coolant for thermal power stations.

What is ‘clean’ water ?

Water quality must be defined according to different uses. Water that is unsuitable for drinking is still usable for washing or many other purposes. In the case of freshwater species, eels tolerate high water temperature, oxygen depletion and low water clarity, whereas trout need a lower temperature, a high level of dissolved oxygen, and clear water.

Washing or wasting?

New Zealanders use about 560 cubic metres of water each per year: that’s 1.5 cubic metres down the drain every day. The United Kingdom is less splash-happy, with 230 cubic metres per person per year. Canadians use a whopping 1,420 cubic metres.


In the early days of settlement, when forests were being converted to pasture, some hill country regions, especially in the North Island, suffered severe soil erosion. This smothered natural waterways and contributed large quantities of sand, silt and clay to rivers, estuaries and coastal regions. Through the early and mid-20th century, eroded soil was the main contaminant affecting rivers and lakes.

Sewage and other pollution

Until the late 1970s the discharge of sewage to waterways caused environmental and health problems. Organic wastes (from meat works, milk factories, wood pulp mills) depleted oxygen in a number of rivers and lakes. These practices are now unacceptable, and most effluents are treated before being discharged into waterways, irrigated onto land, or released into the ocean. As a result, pollution from these sources is now rare, and usually accidental.

Too much of a good thing

In the right place, chemicals such as phosphorus and nitrogen in farm fertiliser are good – nutrients for crops. But when these chemicals are washed into waterways, they upset the natural balance of the water. Also, added chemicals make water weeds grow rapidly and use up oxygen. This can affect fish, lower water quality and clarity, and smother river beds.


By 2000, concern for water quality had shifted its focus from pipes and drains to reducing the impact of more diffuse sources on small streams and groundwater, and the cumulative effect on larger rivers, lakes and estuaries. There have been marked increases in dairy, cattle and deer numbers (but decreased sheep numbers), fertiliser use, animal stocking rates, productivity per hectare, and cropping. These have contributed to the raised levels of nitrate, phosphate, sediment and pathogens in waterways.

Water pollution

Water pollution comes from two main sources:

  • Point sources: single outlets, such as a sewage pipe or a drain.
  • Non-point sources: more diffuse seepage, for instance from underground sources or from small streams flowing into rivers or lakes.

The contaminants can be in solid or liquid form.

Solid materials

These include clay, which clouds the water and reduces the amount of light penetrating it. Heavy deposits of clay or silt may form a layer on gravel river beds, smothering plants and animal life, and releasing unwanted nutrients. Sediments may also carry other pollutants (such as heavy metals, nutrients and pathogens).

Other solid materials are organic forms of nitrogen, phosphorus or carbon (for example, faecal matter and decaying plant matter). These use oxygen and add chemicals to the water, disturbing its natural balance. Solid contaminants are also known as ‘particulates’.

Dissolved materials

These include toxic chemicals that directly affect the health of humans and/or aquatic species (for example, ammonia is toxic to fish). They also include nitrate and phosphate, which boost the growth of water plants such as algae and phytoplankton. In extreme cases, they may trigger blooms of toxic algae. These can clog streams, reduce water clarity in rivers and lakes, and starve fish of oxygen.

Most nitrate in the water from farmland comes from cattle urine, which is concentrated in patches of grazed pasture. Some of the nitrate in urine is washed down through the soil into the water table and eventually into streams and lakes. Phosphate is more readily held by the soil, and less of it reaches the waterways as dissolved phosphate.

Some effluents (from wool scours, timber treatment plants, urban stormwaters) contain dissolved zinc, lead, copper and chromium. These are highly toxic, and make water unsuitable for drinking, and shellfish in river estuaries unsuitable for eating.

Blue babies

In tap water, high levels of nitrate have long been considered a health risk for bottle-fed infants. The babies may suffer infant methemoglobinaemia, in which the blood fails to deliver oxygen. The skin turns blue-gray, and the baby may become irritable or lethargic. Coma and death can result if the condition is not recognised and treated. For this reason, nitrate levels in New Zealand’s drinking water must be less than 11.3 parts per million.

Regional differences

The central North Island’s streams and lakes are naturally high in phosphorus, leached from volcanic rocks, but nitrogen levels tend to be low. Nitrate that enters the water via farm runoff therefore triggers unwanted plant growth. This is a problem in Lake Taupō and other clear-water lakes.

In the Central Volcanic Plateau, runoff from geothermal fields contains arsenic, boron, lithium and mercury, which are toxic.

Elsewhere, such as in Southland, waters are naturally low in phosphate and higher in nitrate. Runoff containing phosphorus from farming, industry and urban areas is therefore causing water quality problems in some Southland rivers.

Temperature and shade

Many of New Zealand’s fish and invertebrates (animals without backbones) have evolved in cool, shaded streams surrounded by forest, and do not tolerate high water temperatures. For example, many stoneflies and mayflies are absent from pasture and urban streams where trees have been felled and summer temperatures exceed 18–19° C.

Regaining the diversity of species depends on restoring shade (by planting on riverbanks) to more than 70%. This is difficult in wide streams, but it is easier in narrow headwaters.

Problem plants and microbes

Disease-causing organisms

Several microscopic organisms cause disease in humans, animals or both. For example, campylobacter, giardia, cryptosporidium and viruses can spread disease via water. They may be picked up through drinking, from recreation such as swimming or boating, or by eating contaminated fish and shellfish.

Diseases range from mild gastroenteric disorders to life-threatening conditions. Following international practice, New Zealand tests the level of Escherichia coli (E. coli) as an indicator of recent faecal contamination in fresh water, and E. enterococci in coastal waters. Water where people swim is monitored to ensure the levels of these ‘indicator’ bacteria comply with legislated standards.


Some of the worst damage to water quality in New Zealand rivers and lakes comes from excessive growths of algae (microscopic plants including diatoms, green algae and cyanobacteria). These algal blooms are usually caused by increased nutrients, but they also need certain conditions, including warm, calm weather. They can reduce water clarity in lakes, and smother river beds, especially in summer.

Rock snot and lake snot

Didymo or ‘rock snot’ (Didymosphenia geminata) has spread throughout several formerly pristine South Island rivers. It forms large blooms on the bottom of rivers and occasionally in lakes, clogging irrigation equipment and covering gravel-bottomed irrigation races, and damaging some recreation and fisheries sites. Slimy 'lake snot' (Lindavia intermedia) has also been found in several South Island lakes.

Milk – and manure

New Zealand’s dairy herds usually graze on pasture for much of the day. They are confined only for milking – 1.5 to 3 hours per day on tracks and in yards. The effluent from this short time comprises up to 12% of the total daily amount of manure. A 400-cow herd produces 132 kilograms of manure during a two-hour stint in the yard.

Aquatic plants

Water weed, water net and oxygen weed are problem plants. Some are rooted in the lake or river bed, others are floating mats that affect the water flow in drains and small streams, and may also block the intake of hydro station turbines. But they do provide food and habitat for some invertebrates and fish.

These plants release oxygen through photosynthesis in daylight, but their respiration in darkness can deplete oxygen in the water. Their eventual breakdown releases nutrients, raising the levels above normal, and may further reduce oxygen in some waterways.


Hornwort (Ceratophyllum demersum) is New Zealand’s most problematic water weed. It can grow up to 10 metres tall, submerged in still water, and can smother other aquatic plants. It is widespread in the North Island, but is currently found only at Timaru in the South Island.

Lakes Rotorua and Taupō, and the Manawatū River

A number of New Zealand’s iconic lakes have been damaged by human activities.

Lake Rotorua

Lake Rotorua has a long history of water-quality problems, including high concentrations of phytoplankton. In the 1970s it was recognised that nutrients flowing into the lake, for instance from sewage, should be reduced. But it was not until 1991 that sewage disposal changed: treated sewage was sprayed on the soil in Whakarewarewa Forest, rather than being discharged into the lake. As a result, phosphorus levels were much reduced.

The nearby Kaituna Catchment Control Scheme saw many stream banks fenced and eroding farmland retired. This lowered the amount of solid material reaching the waterways. More recently, the aim has been to reduce nitrate runoff from farmland, because nitrate concentrations are increasing in the groundwater that feeds springs such as Hamurana.

Lake Taupō

In the Taupō catchment, nitrate contamination of groundwater is a major problem. The regional council aims to reduce nutrient inputs to the lake by 20% by changing land use and improving farming practices.

In 2003 there were blue-green algal blooms in several North Island lakes. Health warnings about swimming were issued at Lake Taupō for the first time. Phytoplankton blooms also occurred in the Waikato hydro lakes and river, and Hamilton’s drinking water supply was contaminated. It is not clear why the blooms were so severe and widespread that summer. But it is generally agreed that if nutrient concentrations in the river had been lower, the blooms would have been less severe.

Manawatū River

During the 1980s fish sometimes died in the lower Manawatū River because of high summer temperatures and severe oxygen depletion, when water levels were low. The cause was the abundance of algae growing on stones in the river bed. This was stimulated by farm runoff and effluent discharges from Palmerston North City and associated industries.

As with several rivers elsewhere in the country, the problems linked to effluent have decreased as factories have closed, or because of better effluent treatment. But nutrients coming from farmland remain high enough to threaten water quality during summer.

Management of water quality

Resource Management Act 1991

Water in New Zealand is protected by legislation, principally the Resource Management Act 1991. It provides a legal framework for using water, managing its quality, and balancing the needs of different groups who affect or use water. For example, fishers and swimmers want the lakes and rivers to be clear, while factory owners need to dispose of waste material.

Regional councils throughout the country monitor water quality, and then decide how to manage land and effluent. In consultation with the local community, a council might decide to maintain high water quality in upland rivers and lakes for recreation and biodiversity, but allow some runoff and waste into lowland rivers and estuaries.

Weeding the water

Harvesting (cutting and removal) is one way to control water weeds. This can improve water quality when unwanted nutrients are removed along with the weed.

Standards and guidelines

The Ministry for the Environment and regional councils, in consultation with the scientific community, has set water-quality standards and guidelines for water and its use.

There are set limits for the physical, chemical and biological factors affecting water quality. For example, dissolved oxygen is kept above 80% of saturation, to protect sensitive fish species. Standards vary according to how the water is used – for example, waters with E. coli counts of less than 260 per 100 millilitres may be fit for swimming or boating, but not for drinking.

Guidelines have the same purpose but are more descriptive (for instance, the absence of algal growths on rocks), and used only as indicators of changes in water quality.


Grass carp fish have proved to be useful at chomping through underwater plants that clog lakes and streams. They can be used for this purpose in New Zealand, if approved by the government and the Fish and Game Council. Grass carp are often confused with koi carp, which can seriously harm water quality. Koi carp can breed in the waterways, whereas grass carp are unlikely to do so.

Farm management

Establishing a link between farm runoff and reduced water quality in rivers or lakes can be difficult. Reducing the impact of farms on waterways has tended to be voluntary, rather than through strict regulations.

Farmers use a number of strategies to minimise effluent loss into waterways. Many dairy farmers now spray effluent from cowsheds onto paddocks, and some have set up wetlands that act as effluent treatment areas. These are managed to be effective over a long period. Unwanted nutrients are filtered and absorbed from the runoff water as it moves through, before it reaches defined waterways. Some regional councils have set limits on the amount of nitrogen fertiliser that can be used on dairy farms, particularly in sensitive catchments such as Lake Taupō.

Both industry and government agencies have a role in managing water quality from farmland.


The dairy company Fonterra Co-operative Group, regional councils, and the ministers for the environment and for agriculture and forestry have a voluntary accord. This aims to minimise the impact of dairying on New Zealand’s streams, rivers, lakes and natural wetlands so that they are suitable, where appropriate, for fish, drinking by livestock, and swimming. This will be done by keeping dairy cattle out of streams, lakes and wetlands, treating farm effluent, and controlling the use of fertilisers.


With the farming industry, the New Zealand Fertiliser Manufacturers’ Association has set up the Code of Practice for Fertiliser Use to reduce the ill effects of fertiliser on the environment. The code has guidelines for farmers and contractors on best practice.

Hononga, rauemi nō waho

More suggestions and sources

How to cite this page: Mike Scarsbrook and Kit Rutherford, 'Water quality', Te Ara - the Encyclopedia of New Zealand, (accessed 15 July 2024)

He kōrero nā Mike Scarsbrook and Kit Rutherford, i tāngia i te 24 o Noema 2008, updated 18 o Hūrae 2016