Chlorine has been added to Lower Hutt's water supply after sample from a nearby reservoir tested positive for E-coli.
About 76,000 people in Lower Hutt are affected.
Wellington Water said E-coli was an indicator for contamination but at this stage there was no way of knowing whether the water supply was contaminated or not.
As a precaution, chlorine had been added to the Naenae reservoir where the positive result was measured, at distribution points below the reservoir, and at the treatment plant that normally supplied unchlorinated aquifer water to 76,000 residents.
The water already in pipes was safe to drink, without the need to boil it, the agency said.
It said the positive result was picked up as part of routine testing.
The results of additional samples taken from the reservoir and the water network would be available on Sunday night.
People might notice a slight smell or taste of chlorine in their water in the meantime, Wellington Water said.
The public artesian wells at Buick Street in Petone and Dowse Square in central Lower Hutt remained open as a source of unchlorinated drinking water.
Wellington Water spokesman Alex van Paassen said another public update would not be made until at least 8pm.
He said positive results happened five or six times a year, and further testing mostly came back with negative results.