The water tank on an Ang Mo Kio block where the tap water was found to have trace amounts of hydrocarbon - the kind of compounds used in anti-mosquito oil - was not tampered with.
But investigations are continuing to determine how the hydrocarbons got into the tank located on the rooftop, Ang Mo Kio Town Council (AMKTC) said yesterday.
The tank is connected to a ground-floor drain where anti-mosquito oiling is done, via a discharge pipe.
The town council will work with the authorities to review the piping system to the water tank, its spokesman said, without elaborating.
It has also submitted a sample of its anti-mosquito oil to national water agency PUB for testing to see if the constituents match those of the hydrocarbons found by the agency.
The spokesman added: "Upon being alerted to (the issue) on Jan 13, we immediately checked the locks to the rooftop access and the water tanks and these were not tampered with.
"We have already conducted thorough inspections on the water tanks in nearby blocks and will also be cleaning the outlets of the discharge pipes as well. We did not find anything that was out of the ordinary during these inspections."
A resident living on the sixth floor of Block 635 Ang Mo Kio Avenue 6 had told PUB on Jan 13 that his tap water gave off an oil-like odour.
PUB said on Friday that while its tests showed the water quality from the mains was good, the tap points contained trace amounts of hydrocarbons.
Hydrocarbons - compounds made of carbon and hydrogen - are the main components of petroleum and crude oil.
The town council flushed the block's rooftop water tanks, which supply water to flats from the sixth to the 12th floors, after the incident. It also hired a plumber to wash and chlorinate the water tanks to remove any remaining traces of hydrocarbons.
PUB provided residents with water bags during the flushing and cleaning operations, which disrupted water supply to flats.
The agency had said on Friday the block's water supply was monitored in the subsequent days and its quality at tap point has returned to normal.
It has also issued circulars warning that spraying insecticides too close to storage tanks during fogging operations can affect the quality of the water inside.