Hamilton City Council is requesting people do not swim near the Pukete Boat Ramp in the next few days and take precautions if they come into contact with river water in the area.
The warning comes after a pipe-fitting failure at the city's Pukete treatment plant may have resulted in a small amount of partially-treated wastewater entering the river.
The council's city waters manager Maire Porter says signs have been erected at the boat ramp following the incident, but indications are any discharge was relatively small.
"Late last night a plug on a pipe-fitting broke away, allowing the partially-treated wastewater to flow into the pump building. The discharge has flowed from the building and through to a stormwater system which leads to the Waikato River.
"While the spill was largely contained on site a portion may have entered the river, although this cannot be confirmed at this time," Mrs Porter said.