The Director of Water Supplies on Monday said officials are investigating what caused a pipe burst that resulted in a water outage in Tung Chung, and apologised to those affected.

Tung Chung was cut off from fresh water for about 12 hours because urgent repairs on a 1.2 metre-diameter pipe were needed following an operational incident near the Siu Ho Wan Water Treatment Works.

Authorities said they originally planned to turn the taps back on at 6am on Monday, but fresh water only returned to Tung Chung at 10am.

Speaking to reporters, Roger Wong apologised to the 120,000 affected residents.

"Because of the site constraints, and also because of the special failure mode of this incident, we take time to investigate and find out the leakage point,” he said.

“The leakage point is also at the bottom of the pipe, so it is hard to carry out the repair works. That's why the progress of [repair] works has been slightly delayed."

Wong added more time was needed to find out the exact cause and a report will be submitted to the Development Bureau.

Built in the 1990s, the burst water pipe is the only one servicing the Tung Chung area.