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.