Selwyn District Council is advising residents in Hororata and the surrounding area who are on rural water supply to boil water for drinking.
A precautionary notice has been issued by the council after test results on Friday showed e-coli levels above recommended levels.
The water is sourced from the Selwyn River.
A council spokesperson says staff have begun chlorinating water and will continue to monitor the intake and reticulation system.
The spokesperson says the council will keep residents informed and will stop the chlorination as soon as no bacteria are present.