Carterton District Council have issued town residents with a 24 hour boil water notice after a positive result for E. coli was discovered.
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Urban residents and businesses in the district have been urged to boil all water from drinking and food preparation after testing confirmed "low-level contamination" in the bores, which supply the 4200 people on the network.
Boiling water before use is the best way to make it safe to drink.
The notice is effective until 3pm, Saturday 13 March.
The source of contamination is being investigated, said Dave Gittings, the council's infrastructure, services and regulatory manager.
"We're confident at this stage that the risk to public health has been minimal, as the council has acted early.
"We have alerted the District Health Board and the local medical centre as a precaution and we are taking all steps necessary to keep our community safe.
"We urge people to follow the boil water instructions and to check on your neighbours and older family members to make sure they know what they need to do to have safe drinking water."
The council reported two other results of the bacteria in December and January.
On both occasions, council staffers locked down two water bores in town water supplies and upped chlorine treatments after testing returned elevated levels of the deadly bacteria.
The 24 hour notice was placed to allow council workers to flush the pipes and change the urban water supply source from the two bores to the Kaipaitangata treatment plant.
The risk of getting sick from drinking the water is low but possible, especially for vulnerable people.
Babies, young children, pregnant women, the elderly and people who have weakened immune systems are more at risk.
Contact your doctor or call Healthline [0800 611 116] if you are in the Carterton area and get diarrhoea, vomiting and/or a fever.
CDC will issue an update on the work to resolve the issue before the notice elapses.