By international standards New Zealand’s coastal waters are relatively clean, although in areas close to larger centres they are more polluted. Little is known about the state of waters beyond the coastal zone. In 1976, local authorities were asked to subjectively rate estuaries in their areas. Of 162 estuaries rated, 38% were regarded as clean, 41% as slightly polluted, 16% as moderately polluted, and 4% as grossly polluted.
Sewage and industrial waste
In the western world the move toward sewage treatment only gained real momentum in the 1980s and 1990s. In New Zealand, as elsewhere, raw sewage was pumped into the ocean – the only human safeguard was to extend the outfall pipes some distance offshore. Signs in these areas warned people not to swim or collect shellfish.
In the mid-1990s a survey found that at least 1.3 billion litres of sewage and water-borne industrial waste were discharged into the sea around New Zealand every day. It also found that large quantities of industrial waste were discharged through sewage outfalls. And it pinpointed that some 80% of marine pollution came from land-based sources, around three-quarters of this being discharged through outfalls. Industrial wastes from wool-scouring plants and freezing works are flushed into the sea – usually via rivers. Urban storm-water run-off (including heavy metals) is another source of pollution. Urban storm-water systems also wash plastic and other debris to the coast.
Since the 1950s, when New Zealand had almost no sewage treatment systems, the country has made slow progress. The last major centres to stop dumping raw sewage into the sea were Wellington and Hutt Valley (in 1998 and 2001 respectively). This change was largely due to the introduction of the Resource Management Act 1991, which placed much tighter restrictions on discharges into the sea. It also gained momentum with the establishment of the Department of Conservation under the Conservation Act 1987. The department became responsible for conserving the coastal marine environment, and demanded higher levels of treatment for all discharges into the sea.
Plastic and other litter is a potential hazard to marine animals. For example turtles, when searching for their staple food of jellyfish, occasionally ingest plastic bags by mistake and suffocate. Plastic items are considered to cause more deaths of marine animals than oil spills, heavy metals or other toxic materials. A year-long study of Auckland’s storm-water discharges found that each day 28,000 pieces of litter, much of it plastic, ended up in the Waitematā Harbour.
Pollution from farming
Agricultural run-off (mainly sediment, fertilisers and nutrients) is a major pollutant of some rivers, and to a lesser extent of estuaries and coastal waters. Sandy estuaries have been building up sediment at the rate of 3–6 millimetres a year, and muddy ones at 2–5 millimetres a year. This amounts to a total increase in the sediment layer over the past 100 years of between 20 and 60 centimetres.
Accelerated erosion affects life in the ocean by covering it in sediment. In some areas of the South Island, reef sponges, kelp forests, weed beds and fish nursery grounds have been lost because of increased sediment. Seagrasses in harbours and estuaries have also disappeared because of declining water clarity, linked to sedimentation.
Conscious of the pollution caused by dairy farms (cow numbers rose 34% between 1994 and 2002), environmentalists signed an accord with the dairy industry in 2003 to try and improve water quality by fencing and planting around rivers.
Most oil spills in New Zealand waters are minor, but they happen reasonably frequently. In 1996 there were 84 reports of marine spills, but it is likely that not all were reported. Only two involved more than one tonne of oil. In 2005 vessels more than 45 metres long were banned from sailing between the Poor Knights Islands and the Northland mainland. This was a pre-emptive move: the area was considered vulnerable to spills, as tankers regularly visit the nearby Marsden Point oil refinery.