Chlorine was added to the water supply for 76,000 Lower Hutt residents on Friday after a routine test from the Naenae reserve come back positive for E coli.

Hutt City Council says the lab which tested the water were unable to confirm why the initial test of the reservoir came back as positive.

The council says chlorination of the water supply will continue until a review of the test results has been completed.

“This is to ensure we are taking the safest course of action,” a statement from the council read.

Wellington Water is investigating the possible source of contamination.