Chlorine is to continue being added to Lower Hutt's water supply as a precautionary safety measure after E coli contamination was detected.
The Hutt City Council said a sample from a bore in the Waterloo well returned a positive E coli indicator test last Wednesday - the third positive result in five months from water sourced from the Waiwhetu aquifer.
A secondary test came back negative, but the chlorination would continue while investigations into the source of the recent positive E coli test results were under way, a council spokeswoman said.
She emphasised the water was safe to drink.
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Authorities say it is possible the phenomenon is linked to the Kaikoura quake's movement.
Recent water quality testing across the aquifer had shown an increasing amount of bacteria activity, which had resulted in the council's decision to continue chlorinating the water.
That has led to the closure of popular artesian well fountains at Buick St in Petone and in central Lower Hutt's Dowse Square, as a precautionary measure because that water cannot be chlorinated.
Greater Wellington Regional Council chairman Chris Laidlaw said the safety of the drinking water quality has not been compromised, but: "We're not taking any chances. We immediately chlorinated Lower Hutt's water supply in response to the recent positive E coli test and we'll continue to chlorinate while we investigate the source of these positive results".
Regional Public Health medical officer of health Dr Stephen Palmer said the department had been notified of the positive E coli test results and was working closely with Wellington Water and Hutt City Council to ensure the public health risk was properly managed.
Lower Hutt mayor Ray Wallace said safe drinking water was of the utmost importance.
"We know many residents really value access to unchlorinated aquifer water but public safety will always be our number one priority. We're working together, taking a cautious and vigilant approach, and will continue to do so throughout the investigation," said Wallace.
Wellington Water acting chief executive Mark Kinvig said the cause of the positive test results was not known yet, but it was possible it was related to the November 2016 Kaikoura quake.
"We're carrying out investigations into the source but this is expected to take many months to complete."
About 70,000 Lower Hutt residents receive unchlorinated water, and will be able to taste and smell the addition of chlorine to their drinking water.