Waterfront mansions are in danger of being washed away as Sydney beachside suburbs are inundated in thick sea foam.

Memories of the June 2016 storm have come flooding back for Collaroy residents on Sydney's northern beaches as the weekend-long wild weather continues to batter the coast.

Residents living along the beach between Collaroy and Narrabeen woke on Monday to significant erosion and a massive build up of sea foam with up to 25 metres of sand washed away.

Photos shows waves of a dirty-coloured sea foam coming within metres of beachfront mansions.
Further beach erosion damage was expected later on Monday morning.

'Basically, the next few hours will be critical,' University of NSW coastal researcher Mitchell Harley told the Sydney Morning Herald.

'After that, the risks will drop significantly.'

He warned another threat of further beach erosion could emerge next weekend.

'Plenty of sea foam going into backyards, but erosion risk dropping. Impact to houses spared, ready to go again for next weekend's forecast massive waves and already eroded beaches,' Dr Harley later posted on Twitter.

The erosion damage already caused could possibly take months to repair.
The sea foam was mid-calf deep in one Collaroy street overlooking the beach.