New national water quality figures show a mixed picture in rivers and lakes, with E. coli getting worse at many river sites and overall lake health remaining poor, even as phosphorus levels trend down.

Stats NZ today reported that total phosphorus decreased at 59 percent of monitored river sites between 2005 and 2024. “Phosphorus occurs naturally at low levels in rivers. Lower phosphorus levels generally mean less risk of excessive algal growth,” environment statistics spokesperson Tehseen Islam said.

Over the same long-term period, E. coli trends increased at 38 percent of monitored river sites and decreased at 21 percent, indicating more sites are moving in the wrong direction for contamination from animal or human waste. “E. coli is a sign that water may be contaminated with animal or human waste, which can raise health concerns,” Islam said.

Nitrogen results were mixed. Between 2020 and 2024, 68 percent of monitored river sites sat above the default guideline value for total nitrogen in the Australia and New Zealand guidelines for fresh and marine water quality (ANZG), a level associated with a higher likelihood of negative effects on river ecosystems. Looking over 2005–2024, similar proportions of river sites showed increasing and decreasing trends for total nitrogen.

Lakes remain under pressure. Between 2020 and 2024, 73 percent of monitored lake sites were rated poor or very poor on the trophic level index (TLI), a New Zealand tool that assesses ecological health based on nutrient levels and algal growth. From 2005 to 2024, TLI trends declined at 36 percent of monitored lake sites.

The trend analysis covers hundreds of river sites—495 for phosphorus, 453 for nitrogen, 543 for E. coli—and 44 lake sites for TLI. Stats NZ says the trends highlighted here have at least a 90 percent likelihood of being real change rather than random variation.

More detailed results, methods and limitations are available in the environmental indicators released today. The indicators are published by Stats NZ as part of a joint environmental reporting programme with the Ministry for the Environment.