Residents of Martinborough in Wairarapa are being warned they could have blue or black water coming out of their taps as the water supply is disinfected.
E coli was detected in parts of the town's water supply last week.
Residents have been told to boil water or take it from emergency water supplies available.
The South Wairarapa District Council said the levels of E coli were low, but it was going to chlorinate the system as a precaution.
As a result, water out of the tap could be strongly discoloured.