The levels of the potentially harmful E.Coli have risen in Lake Rotoroa at Hamilton Lake Domain.
This is a common occurrence at this time of year, as the lake's resident duck population increases significantly with birds seeking sanctuary during duck shooting season.
The spike in the duck population results in a rise in the E.Coli levels, as more faeces enters the lake.
Visitors to Hamilton Lake Domain are asked not to consume or cook with any water from the lake, not to consume any fish caught in the lake, and not allow pet animals, particularly dogs, to swim in the lake or consume any of its water.
Hamilton City Council has arranged for warning signage to be placed around the lake edge and will continue to monitor the situation.