Levels of E coli bacteria have increased at 70 percent of monitored freshwater sites in Taranaki over the past 10 years.
Waiwhakaiho River at New Plymouth has a permanent health risk warning not to swim there posted. Photo: RNZ / Robin Martin
The finding is included in the Taranaki Regional Council's State of the Environment report for 2022.
The bacteria are usually found in the guts of warm-blooded animals and people, and are an indicator of faeces in the water.
The report finds that swimmer safety has been affected by water quality at the 37 sites that are monitored between November and March each year.
They were found to be unsafe for swimming 35 percent of the time, while beach sites were unsafe 10 percent of the time.
The report stated that 42 out of 175 lake surveys showed toxic algae present at levels that meant they were unsafe for swimming.
Permanent health risk warnings not to swim have been posted at three urban sites in the lower reaches of Te Hēnui Stream, Waiwhakaiho River at New Plymouth and Waimoku Stream at Oākura.
The council says progress on improving water quality has been made, but there is more work to do.