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.