Health authorities say high levels of E.coli identified at Kaiate Falls sparked a warning this week for people to stay out of the water.
The bacteria E.coli can cause a raft of illnesses.
Toi Te Ora Public Health Service medical officer of health Phil Shoemack issued the permanent health warning after ongoing water quality testing by the Bay of Plenty Regional Council over several summers showed high levels of E.coli.
Dr Shoemack said the regional council had done extensive testing of the wider catchment upstream from Kaiate Falls, but as yet had failed to identify the exact source of the contamination.
It was likely to have come from wild or farm animals or possibly someone's septic tank.
The regional council was working with nearby landowners to identify the source and reduce any potential agricultural sources, he said.
Dr Shoemack said that, while there had been no reports of people being treated for any illnesses or hospitalised after swimming at the falls, the contamination risk had reached unacceptable levels.
"This is very much a preventative warning, as the level of contamination has reached a level which is above the acceptable NZ threshhold," he said.
Dr Shoemack said the risk of illness was high, especially if a child put their head underneath the water or someone had a laceration or open wound.
Contamination of recreational waters can cause a number of illnesses, such as gastroenteritis, which is a diarrhoea and vomiting illness, or inner ear and throat illnesses, he said.
In January, Kaiate Falls recorded unsafe levels of faecal contamination and there were reports of six people who suffered vomiting and diarrhoea after spending time in the falls and on other waterways.