Two dead rats have been pinpointed as the source of contamination at a water reservoir in the township of Eureka near Hamilton.
Waikato District Council says the rats were found after the reservoir was drained for cleaning.
After the reservoir has been cleaned it will be refilled and the water tested before service resumes.
The council expects water supplies to return to normal by Friday.
In the meantime, residents are being reminded to boil all their drinking water.