Up to 25 metres of beach at Collaroy and Narrabeen has been swept away by huge waves generated by the east coast low that brought a deluge to Sydney at the weekend.

The storm weather brought “abnormally high tides” and strong winds and the city’s heaviest rainfall in up to two decades, the Bureau of Meteorology said.

Drone footage showed waves sweeping up to the foot of houses along the beach where a similar storm in 2016 destroyed a private swimming pool and caused serious erosion.

A coastal researcher, Prof Mitchell Harley, from the University of New South Wales, said properties along the beach had been spared serious damage on Monday but the threat would resume in coming days when more storms were forecast.

Harley told Guardian Australia there were two realistic ways to protect beaches and nearby properties.

“First, hard structures are built to defend against the effects of waves,” he said.

Seawalls made from concrete, wood, steel or even boulders can slow the movement of sand away from the beach, but not prevent it completely.

“Eventually, you end up with no beach,” Harley said.

“The other option is beach nourishment, where you inject large amounts of sand, up to hundreds of thousands of cubic metres of sand, straight into the beach on a regular basis.”

But the sand needed for beach nourishment is mined at a depth of 20 to 30 metres, and Australia has neither the policies to enable that nor the equipment to conduct it.

The Bureau of Meteorology issued another warning at midday on Monday for abnormally high tides, damaging winds, damaging surf and heavy rain for much of the NSW coast, including the metropolitan area.

SEVERE WEATHER WARNING: for Abnormally High Tides, Damaging Winds, Damaging Surf and Heavy Rain for the South Coast and parts of Northern Rivers, Mid North Coast, Hunter, Metropolitan, Illawarra and Snowy Mountains Forecast Districts. Warning: https://t.co/50gv5lymdD pic.twitter.com/wvJIXOB51o — Bureau of Meteorology, New South Wales (@BOM_NSW) February 10, 2020

Harley warned that south Palm beach, Coogee and Stockton beach in Newcastle were at particular risk of erosion, as well as Collaroy.

Stockton beach was significantly affected on Monday, with “dunes eroded back several metres, leaving caravan park teetering on the edge”, Harley wrote on Twitter.

Erosion at Stockton has forced a daycare centre to close and put several other beachside properties under threat.

The storm waves have also affected nearby areas between Manly and Shelly beach, with waves crashing over the walkway. Harley strongly advised against outdoor activities such as bike riding there.

The severe flooding along the east coast prompted the Insurance Council of Australia to declare the sixth catastrophe warning in five months.

By 7am AEDT on Monday, insurers had received 10,000 claims estimated to be worth $45m, the insurance council said.

Storms have hit south-east Queensland and NSW coastal regions in the past week, also causing damage several hundred kilometres inland, and in the ACT.

Most claims have come from Queensland and coastal NSW for property damage caused by storm runoff, flooding, strong winds and heavy rain.