Levels of potentially harmful E.Coli have risen in Lake Rotoroa at the Hamilton Lake Domain.
The Hamilton City Council said it was common at this time of year because more birds were on the lake, escaping the duck shooting season.
E.Coli is caused by more faeces getting into the lake.
The council said visitors to the domain should not consume fish caught in the lake, consume or use water from the lake for cooking, and not allow animals into the lake.
Warning signs will be put up around the lake edge.