Residents on the Lake Coleridge water supply in Canterbury are being told to boil their water because its usual ultra violet treatment may not be working.
The Selwyn District Council said on Thursday that routine monitoring of the water supply has found the UV treatment could be ineffective because the water is very turbid.
No E. coli organisms have been detected, but the council said it can't guarantee it's safe to drink.
The water supply has been chlorinated, but only intermittently, and may not be effective all the time.
The boil water notice is likely to be in place for several days.
