People in Lower Hutt might notice the addition of "a slight smell and taste of chlorine" in their drinking water.
Wellington Water has added the chemical to the water supply, after a sample from Naenae reservoir tested positive for the bacteria e-coli.
The water treatment organisation says the addition of chlorine is precautionary.
Positive tests for e-coli do occur from time to time, but "this does not necessarily mean the water is contaminated," Wellington Water said in a statement.
"We will always introduce chlorine to part or all of the network after a positive result, until re-tests confirm there is no risk of contamination."
Around 76,000 Lower Hutt customers normally receive an unchlorinated water supply. The water comes from rivers and aquifers. When water is scarce, lakes provide additional water to the area.
Members of the public who wish to drink unchlorinated water can use the public wells at Buick St in Petone and Dowse Square in central Lower Hutt.
There is no need to boil water already in the pipes, Wellington Water advises.
Results from further testing should be available after 8pm on Sunday.