The chairman of the Drinking Water Safety Advisory Committee said on Thursday black particles found in tap water at two Fanling housing estates may have originated from upstream water pipes.

Laboratory tests carried out by the government are under way to help find out where the particles are coming from, and the results are expected to be out this week.

Speaking on a Commercial Radio show, Chan Hon-fai said a lab test by the Education University commissioned by a media organisation suggested that the black particles are asphalt, which is used as a coating to prevent rust.

But as the material was banned from use in Hong Kong pipes years ago, he said asphalt should not be found in the water system of the two relatively new developments.

The first batch of tenants moved into the two affected housing estates – Queen's Hill Estate and Shan Lai Court – in 2021 and 2022, Chan pointed out.

"My deduction is if there's no asphalt for pipes inside Queen's Hill Estate, then it has to come from external sources, possibly upstream public pipes," he said.

"These asphalt particles may have accumulated in the underground pipes for some time, and after a certain period, I don't know why, they appeared in water tanks, or even in the tap water of households."

The engineer also noted that the toxicity test results carried out by the department came back negative.