The Bay of Plenty Regional Council is warning people not to swim or play in places where stormwater drains, ponds or sewers empty into the region's rivers, lakes and beaches, following a long period of hot, dry weather.
Regional Council water scientist Paul Scholes said because of the weather, many stormwater drains and ponds had had little or no flow leaving the water at risk of bacterial contamination.
"The water is sitting stagnant, and the high temperatures we've been having increases bacteria and a build-up of other contaminants which could be a health risk to both people and pets."
Water at Harrisons Cut outfall onto Papamoa Beach, where families often swim and paddle, was being regularly monitored for health risks, he said.
Mr Scholes said Tauranga City Council deepened the channel across the beach to improve water flows but people should not swim or paddle there.