Glentunnel residents are being advised to boil water after the Selwyn River flooded last week.
The Selwyn District Council said unacceptable levels of e.coli were in the water as a result of heavy rain last week.
The organism can cause vomiting and diarrhoea.
The council were chlorinating water at the time, but had increased the dosage after testing showed e.coli was still present in affected areas.
If people wish to disinfect private tanks, they can contact the council for further information.
The council said it would continue to monitor the intake and reticulation system and keep residents informed.
Chlorination would stop as soon as soon as no bacteria are present.
- The Press