Hororata residents are asked to boil their drinking water.
Selwyn District Council has issued a warning for those who use the rural water supply after test results showed high levels of e-coli.
People are advised to visit their doctor if they have any symptoms, including vomiting and diarrhoea.
The council has begun chlorinating water in the area.
The council will continue to monitor the intake and reticulation system and keep residents informed.
Chlorination will stop as soon as soon as no bacteria are present.