Corrosion caused burst water main

A corroded section of a water main the size of a tissue box in Sydney's northern beaches is to blame for the flooding of a mansion-dotted hillside and the sinking of a fire truck.

Residents at Bilgola, near Avalon, awoke on May 13 to find mud throughout their homes after a cast iron pipe burst and caused a landslip.

A fire truck rushing to the scene also became stuck in a sinkhole after water damage caused part of the road to collapse.

Sydney Water confirmed on Tuesday a cement-lined water main running through The Serpentine gave way due to a minor defect in the pipe's protective coating.

They said investigators had found a corrosive patch measuring 20cm by 10cm, slightly smaller than the size of a standard tissue box.

As a result of the defect, the pipe became corroded by soil, causing a section of it to break away.

The state-owned utility has apologised to the residents affected by the burst water main, and reassured residents that the water main is unlikely to cause further problems.