Chlorine has been added to Lower Hutt's water supply, in another E. coli scare.
Water for around 76,000 residents is being treated after a reservoir in Naenae tested positive for the bacteria.
Chlorine is being added to piped water in areas that do not usually receive chlorinated water, Hutt City Council has advised.
"For extra security, chlorine is also being added at the treatment plant that supplies normally unchlorinated aquifer water to the majority of Lower Hutt residents (except Wainuiomata, Stokes Valley and Manor Park - these areas are already chlorinated)," it said in a statement on its website.
Water already in the pipes is safe to drink, without the need to boil it, Wellington Water said.
E-coli is an indicator for contamination, Hutt City Council said in a statement on its website.
"At this stage there is no way of knowing whether the water is contaminated or not," it said.
"So as a precaution, Wellington Water staff are adding chlorine to the reservoir and at distribution points below the reservoir."
Results of additional samples would be available later.
The public artesian wells at Buick St in Petone, and Dowse Square in central Lower Hutt remain open as a source of unchlorinated drinking water.
Wellington Water will carry out tests on multiple samples at other reservoirs and points in the water network, and provide updates to Lower Hutt residents as soon as those results come in, the statement said.
The test result comes just days after water in Napier came up positive for E. coli.
Napier's water is being chlorinated for the rest of the long weekend, as a precaution.