Tests have found unsafe E.coli levels in a Canterbury council's water supply, and some residents are being advised to boil their water.

The Selwyn District Council said test results showed E. coli levels were above those recommended in the Southbridge area water supply, which posed a health risk to anyone drinking it before it is boiled.

About 350 households get their water from the Southbridge water supply.

Those affected were advised to boil or treat all tap water before drinking, brushing their teeth or using it for preparing food.

E. coli is a bacteria that can cause serious illness. Anyone who becomes sick should visit their doctor.

The council said it would notify residents through its website when the boil water notice was lifted, but it was likely to be in place for several days.

- RNZ