Residents in the Canterbury town of Rolleston are being advised to boil water after high E. coli levels were found in the water supply.
The affected well has been switched off from the main Rolleston supply to prevent further contamination.
Selwyn District Council water services manager Murray England says the council has begun chlorinating the water.
He says about 8000 residents may have to boil their drinking water for the next few days.
Mr England says once there are three consecutive days of clear water samples then the boiled water notice will be removed.
Just two weeks ago, the council advised people in Sheffield to boil water after a similar problem was found in that town's supply.