People on the Arthur's Pass water supply are advised to begin boiling water, because heavy rainfall has made the source more turbid.
The Selwyn District Council says while no E.coli has been detected in the supply, the normal treatment method may not be effective, and it cannot guarantee the water is safe to drink.
People are advised to keep boiling their water until otherwise notified, which could take several days.
Anyone with symptoms of sickness should visit their doctor as soon as they are able to.
The council says it will continue to monitor the intake and reticulation system and keep residents informed of any updates.