A huge low pressure system that brought flooding and strong winds to New South Wales over the weekend has combined with a king tide to bring widespread coastal destruction, forcing people to flee homes.

Key points: Residents of about seven houses and a unit block at Collaroy moved

Residents of about seven houses and a unit block at Collaroy moved Backyards washed into ocean as waves continue to erode coast

Backyards washed into ocean as waves continue to erode coast 21 flood warnings in place statewide, after 9,000 calls for help

As towns from Lismore in the state's north to Wollongong and south-west Sydney dealt with flooding, on Sydney's northern beaches, a number of houses and a unit block were evacuated when the wild weather battered the coast.

People were removed from about seven houses and the unit block at Collaroy as large waves up to eight metres high continued to erode the coast, crossing 50 metres over the coastal road, police said.

A number of properties have been damaged with backyards washed away, police said.

"One of the properties along that strip actually lost an entire in-ground pool which had moved above five metres out to sea," Inspector Jason Reimer said.

Narrabeen resident Adam Deere-Jones and his family stayed with relatives after the Narrabeen Lagoon burst its banks and threatened to flood his home.

"It was pretty scary. It all happened pretty quickly," he said. "We woke up about 8:00am and it was just coming across the road.

"Then within an hour it was sort of up to these front steps here. Then SES and cops came around and said 'time to evacuate'."

On Sunday night, a number of sinkholes opened up along the foreshore and emergency services worked into the night to save the homes.

One local resident, David, said the erosion was immense.

"There is no beach at Collaroy," he said. "I wouldn't be calling it Collaroy Beach anymore, I'd be calling it Collaroy Point."

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Sydney's eastern suburbs were also badly hit, with Coogee's historic surf club suffering extensive damage.

The eastern wall of the club, which has stood since 1907, has collapsed and fire crews are trying to secure the first floor.

"We've had significant damage in our boat shed, we've lost almost all of our lifesaving gear," the club's president, Mark Dopell, said.

Elsewhere in the state two people were found dead after floodwaters swept away their cars.

Police recovered the body of a man from floodwaters at Leppington, in Sydney's south-west, while another man's body was found in Bowral in the Southern Highlands.

In the ACT, the body of man who was trapped in a car and swept away in floodwaters near Canberra has been recovered.

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Flooding hits across the state

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Across NSW, 21 flood warnings were in place and thousands were without power on Monday.

In Lismore, the river peaked at 9.1 metres, damaging roads, parks and infrastructure, although newly installed levees on the Wilson River prevented more widespread damage to property.

In Sydney's west and south-west, residents were evacuated from low-lying areas near the Georges River, Chipping Norton and Woronora due to flooding and several roads in the area were closed.

In Picton, about 30 businesses were flooded after two metres of water swept through the town, coating properties in mud.

Communities near the Nepean River in Sydney's west and south west are also watching river levels with several properties affected in Camden.

Phil Perkins, from the Bureau of Meteorology (BoM), said there are still areas of concern in Sydney and in the state's far south.

This stairwell on Wamberal Beach on the NSW Central Coast had its foundation ripped away by waves.. ( ABC News: Brad Storey )

"We've got major flooding or a major flood warning for the Nepean and a minor but pushing major for the Hawkesbury as well as the Georges River," Mr Perkins said.

"We've got moderate in the Bombala river in the Snowys, Moruya as well as the Shoalhaven, Queanbeyan and Molonglo."

On the Central Coast, Terrigal was badly hit by the tidal surge when the rising lagoon forced the evacuation of a motel.

A large number of main roads, including across the Sydney metropolitan area, are not in use.

By Monday afternoon there had been about 10,000 calls for assistance statewide, the State Emergency Service said.

Damage after yesterday's storm at Collaroy Beach. ( ABC News: Amanda Hoh )

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The heaviest rainfall in Sydney's metropolitan area was Ingleburn in the south-west, with 379 millimetres falling.

On the South Coast, Jervis Bay received 400mm while Robertson in the Southern Highlands received 618mm.

"I grew up in a town called Parkes in the Central West and that's our annual rainfall in a weekend," said the Bureau of Meterology's Rob Taggart.

"Its quite unbelievable."

In Coffs Harbour, huge waves that caused damage to the marina on Saturday continued into Sunday.

Waves reaching 11 metres bashed the pontoon and sank two more yachts on Sunday night.

The system is currently moving southwards, with the clean up starting in Sydney and the north coast.

Sorry, this video has expired Picton's main street floods

James Morris from the State Emergency Service said the focus was shifting to the Illawarra and South Coast region, but problems still remained for other areas.

"Most of the North Coast, Mid North Coast and Sydney will see very little rainfall," he said.

"In saying that we are obviously reminding people [that] there is still the threat of rising floodwaters in numerous areas."

People inspect the storm damage after a king tide combined with a low pressure system to batter Collaroy Beach. ( ABC News: Rachel Pupazzoni )

Schools keep doors closed

Parents are also being advised to check their local schools, with many affected, including on Sydney's northern beaches.

Narrabeen Lakes Public School is closed today due to storm damage.

Other schools open today with limited supervision include Cromer Public School and Marrickville High School in Sydney's south-west.

Albion Park Public School and Kira High School have also been affected.

Road closure at Bomaderry, some local roads remain closed in the Shoalhaven. ( ABC News: Liv Casben )

Historic wharf damaged on NSW south coast

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A huge northerly swell has moved one of the pylons under the historic Tathra Wharf, which was built in the 1860s, causing the pedestrian platform to buckle and break away from its support.

Bega Valley Shire Council general manager Leanne Barnes said engineering teams would assess the damage once conditions improve.

"The wharf platform structure has slipped off at least one of the pylons," Ms Barnes said.

"It is therefore lifted. It is obviously too dangerous to have any structural engineering assessments done at this point.

"The area is absolutely off-limits to people while we go through and get an assessment of the heritage wharf."

The ward, which was in use from 1862 to 1952, was recently restored by the National Trust, Department of Planning, and local residents.