Wellington Water managers will meet today to discuss the results of tests on Lower Hutt's water, which came back negative for E. coli.
On Saturday, the company added chlorine to the water supply after a sample from the Naenae Reservoir tested positive for the bacteria.
It said tap water was safe to drink and there was no need to boil it.
Company spokesman Alex van Paassen said even though last night's test results were negative, it was not known what caused the initial positive reading.
Today's meeting would review all the results and consider how long to continue adding chlorine. About 76,000 people in Lower Hutt were affected.
Wellington Water said positive tests came back five or six times a year, and further testing usually came back negative.
The positive result was picked up as part of routine testing.