The Selwyn District Council is advising residents in the Malvern Hills Rural Water Scheme, including Dalethorpe, Hartleys Rd and Glentunnel and the Sheffield area to boil their water.
Testing on Friday has shown unacceptable levels of e.coli, an organism which can cause health issues.
The Selwyn District Council has begun chlorinating in affected areas.
Residents will be notified as soon as the water returns normal results.
