Two small towns in the Hauraki district have been told to boil water after the water supply was compromised during bad weather this week.
A Hauraki District Council spokesperson said it was "playing it safe", with about 200 households affected in Mackaytown and Kaimanawa.
Council chief executive Langley Cavers said Mackaytown's water contained high levels of sediment, which could reduce the effectiveness of chlorine in killing bacteria.
E coli has been detected in Kaimanawa, where the water comes from a creek.
"Boil the water for drinking, for brushing your teeth and if you're using water preparing food you should use boiled water for that," Mr Cavers said.
"Once the turbidity levels come down - this rain disappears - we'll be in a poisition to re-evaluate and hopefully lift that boil water notice."
Mr Cavers said Mackaytown should be connected to a safer water supply by the end of the year.
Mackaytown is expected to be connected to the recently upgraded Paeroa water supply in the next six months.
The council has more sophisticated water treatment plants in Paeroa, Waihi, Hauraki Plains and Waitakaruru, with several contaminant-preventing barriers, but its rural networks relied on a single-barrier chorination treatment, the council said in a statement.
A chlorine dosing fault had been discovered and repaired in the Kaimanawa supply, and the network was being flushed until all parts had enough chlorine, the council said.