Pahiatua residents will have to boil water for at least three days due to bacterial contamination.
The Tararua District Council issued a boil water notice after test results showed the presence of E. coli in the township's drinking water.
The community's water is also being chlorinated.
Council workers are going door to door around homes and businesses, and all schools, rest homes, medical centres, sports centres and day-centres have been phoned.
Tararua Mayor Roly Ellis said they would not know the extent of the contamination until tomorrow.
He said it was a precautionary measure because they felt they needed to stamp on it straight away.
Ellis said the boil water notice will remain for at least three days.
It comes after more than 5000 residents of Havelock North and surrounding communities have been affected by E. coli contamination in the water supply.
Residents there have been boiling water for the past three weeks.