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Analysis of national river water quality data for the period 1998-2007

Publication date:  May 2010
Publication reference number:  CR 88

This report is one of two prepared for the Ministry for the Environment by NIWA, which update national indicator data for river water quality in New Zealand and help provide the evidence base for policy and decision makers.

This report provides information on the state of New Zealand’s rivers and recent trends in river water quality. The report includes data from about 600 regional council and National River Water Quality Network sites. This report updates an earlier report based on the national network only for the 19 year period, 1989-2007.

The December 2010 version of the report replaces an earlier version published in May 2010.  The key difference is that the latest version provides more detailed commentary on the methodology used, specifically giving a more comprehensive assessment of the uncertainties that arise from the estimation of flows.

The second report models water quality for all river reaches in New Zealand using water quality data for the 600 regional council and national network sites and a range of catchment features such as climate, geology, topography and land cover.

Key findings

State of river water (2003-2007)

  • The state of water quality and recent trends in New Zealand’s rivers is highly variable around the country.
  • Areas where river water quality is significantly deteriorated include lowland areas of Northland, Auckland, Waikato, the east coast of the North Island, Taranaki, Manawatu-Wanganui, Canterbury and Southland.
  • Rivers in urban and rural areas generally have poorer water quality compared to native forest. Rural areas in particular are under increasing pressure as land use intensifies. Results for 2003-2007, indicate that median levels of total nitrogen are 5 times worse in pasture and 9 times worse in urban than in areas of native forest.
  • Also, on average, between 2003 and 2007 the water was half as clear in areas of pasture compared to areas of native forest. Clear water is important for aquatic life and recreation.
  • Based on the river water quality data for 2003-2007, over 50 per cent of sites in Auckland, Waikato, Canterbury and Southland regions had median total nitrogen levels that exceeded the New Zealand guideline value and over 50 per cent of sites in Northland, Auckland, Waikato and Southland had median total phosphorus levels that exceeded the guideline value.

Recent trends (1998-2007)

  • A previous study covering a longer time period (1989 – 2007) showed nationally that nutrient levels increased (deterioration in water quality) and clarity improved.
  • However, the recent findings show that changes in water quality at the national scale over the 10 years are varied. Between 1998 and 2007 national water quality deteriorated for total nitrogen, total phosphorus, conductivity and clarity but improved for bacterial and ammoniacal nitrogen levels.
  • There were, however, many river sites that showed no significant change between 1998 and 2007. For example, total phosphorus concentrations showed no significant change at 77 per cent of monitored sites around the country.
  • The reports also show sites where there is declining water quality. For example, 23 per cent of monitored sites showed a significant increase in total nitrogen levels between 1998 and 2007 and 26 per cent of sites showed a significant decrease in clarity. Both of which are a deterioration in water quality.
  • Between 1998 and 2007 national water quality deteriorated for clarity, total phosphorus, total nitrogen, oxidised nitrogen and conductivity in rivers with catchments dominated by pastoral land cover.