Sydney councils and the state government are facing a multibillion-dollar bill to strengthen and maintain sea walls around the harbour and beaches in the face of rising sea levels.

But despite the threat to low-lying areas, housing densities are increasing in high-risk areas. New developments are being constructed in areas that depend on sea walls around Manly lagoon, at Rushcutters Bay, Double Bay and along the Parramatta River.

Eroded sand: Ashleigh Smith (left) and Michaela Norris play at City Beach, Wollongong. Credit:Sylvia Liber

If the 82-centimetre rise in the sea level predicted by the Intergovernmental Panel on Climate Change is realised by 2100, coastal developments the length of the NSW coast will be at risk.

In Sydney, the hundreds of kilometres of walls around the harbour will become increasingly vulnerable to stronger wave action in deeper water. These waves can scour out the footings and cause ''overtopping'', when waves slop over a retaining wall and erode it from behind.