Emergency crews are searching for two people whose car was washed away by flash flooding near Maitland in the Hunter Valley, as gale force winds and heavy rain continue to batter Sydney and surrounding areas.

The SES has also ordered the immediate evacuation of homes in parts of Milperra, in Sydney's south-west, where the Georges River is predicted to flood.

Forecasters say the worst of the conditions have passed but a severe weather warning remains in place for the Illawarra region, south of Sydney.

Two drivers died in a crash on the New England Highway near Singleton, and two passengers were flown to hospital with serious injuries, authorities said.

Meanwhlile rescue teams are searching for two people, one thought to be an elderly woman, who went missing after their car was swallowed by rising floodwaters at Cessnock Road in Maitland this morning.

Four people who tried to rescue them had to be rescued themselves by emergency services after getting into difficulties.

"We have potentially two more fatalities," SES Deputy Commissioner Steve Pearce said.

"Hopefully it may come to a good outcome, but we have grave concerns there."

Three people died in Dungog, north of Newcastle, yesterday as homes were washed away by floodwaters after 300 millimetres of rain hit the area in less than 24 hours.

Emergency services have described the weather conditions as "devastating and relentless".

Mr Pearce said crews were working hard in "horrendous" conditions to clean up the damage from yesterday's storms, and the wild weather was expected to continue as another low-pressure system moved in from the coast.

SES helicopters have been able to take to the air for the first time in 36 hours to survey the damage.

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The helicopters are being used to take supplies in and out of the worst-hit areas including Dungog, where fire and rescue authorities say four houses have been washed away and up to 80 severely damaged by flooding.

Floods also damaged houses in the town of Greta, where one local man described how he clung to a tree as his house was swept off its stumps yesterday.

In the northern Sydney suburb of Narrabeen, 10 homes were evacuated this morning after authorities became concerned at least one could collapse.

Heavy rain washed large amounts of foundation from under the wooden structure in Nareen Parade.

More than 225,000 homes are now without power and almost 100 people have been rescued from floodwaters.

Emergency services have responded to more than 10,000 calls for help as a result of flash flooding, fallen trees and downed powerlines.

Extra crews from across New South Wales and Queensland are coming to assist in what Ausgrid says is a significant backlog of jobs that will take "a few days" to get through.

Premier Mike Baird urged storm-hit communities to "hang tough", saying the weather was "still very dangerous" and Maitland and Dungog were declared natural disaster areas.

"We're seeing a continuation of the wind, rain, flash flooding, and whilst it is still dangerous, the positive is that there is some easing. But we still have a lot to get through in the next 48 hours," he said.

"To give you a sense of the size and scope - in Dungog there's more rain that has come down in the last 24 hours than they have seen in a 24-hour period for the past century."

Part of the town of Maitland, including the New England Highway, was flooded. ( ABC News: Melissa Clarke )

In Sydney, the situation eased a little this morning. Ferries have resumed between Circular Quay and Manly, after the Harbour Master shut down operations yesterday because of rough conditions around the Heads.

A severe weather warning remains for heavy rain and gusty winds for the Illawarra region, but has been lifted in Sydney and the Hunter region.

Forecaster Sean Carson said the winds had halved in strength since yesterday.

"We've seen a gradual easing over the last 36 hours, and particularly this afternoon, now that the low pressure system starts to move to our south and weaken," he said.

The Carnival Spirit cruise ship, which was unable to enter Sydney Harbour yesterday because of heavy seas, docked this morning at Circular Quay with more than 2,000 people on board.

Domestic flights from Sydney Airport have been delayed by up to an hour and some international flights are being diverted to Melbourne and Brisbane.

On the rail networks, the Hunter Line is closed and there are partial closures on the south coast, Central Coast and Newcastle lines.

The Manly Dam began overflowing this morning but the SES says residents downstream are not in danger.

Police and Emergency Services Minister David Elliot said people needed to remain cautious and stay away from floodwaters, with almost 100 flood rescues overnight.

"If there are floodwaters — and I highlight that we have had 93 rescues overnight from floodwaters — people shouldn't cross them," Mr Elliot said.

"Do not attempt to cross bridges that look to be victims of floodwaters, do not attempt to cross roads that have been crossed by floodwaters and if there is a river that is about to break its banks, don't go near it. This is the most important part of the messaging today."

Extra SES crews were brought in from interstate to help volunteers in the Hunter region.

Sorry, this video has expired Footage taken from a Westpac Rescue Helicopter shows severe flooding in the Hunter Valley.

The SES's Andrew Richards said crews conducted six flood rescues around Maitland overnight, including for a large herd of cattle.

"A farmer had identified that some of his cattle were up to their knees in floodwater and contained by a fence so he called upon the SES in a flood boat to go out and cut the fence so the cows could escape and move to higher ground. It was quite a herd of cattle," Mr Richards said.

Meanwhile, schools and TAFE campuses in parts of NSW have been closed.

"Cyclonic" conditions yesterday wrought havoc in the Hunter, Sydney, Central Coast and Illawarra regions.

Dungog local police commander Jeff Loy said the deaths of three elderly people in the Dungog area occurred in different locations.

"Two males and one female all perished in different circumstances," he said.

"The police are investigating the cause of those deaths.

"I understand there has been footage also of a house going down the river. These people are not related to any of those houses that have been taken off their piers," he said.

Debris lies strewn across a property at Dungog. ( @FRNSW )

Dungog resident Tracy Clifton said her friend barely made it out of his house alive.

"I was speaking to one of the fellows who live there and he said he saw the water coming up across the road and decided to get out," Ms Clifton said.

"He got out the family and the kids and he said when he went back in the water was already up to his waist so he thought he'd better get out as well.

"And it was only a matter of minutes and his house was gone, was taken down the stream."

Dungog Shire Council Deputy Mayor Tony McKenzie said the town was "very quiet and sombre".

A man inspects a damaged car amid debris in Dungog. ( Facebook: Fire and Rescue NSW )

Mr Pearce said yesterday's storm was like nothing he had seen before.

"I haven't seen a storm of this magnitude in my time here at the SES and, indeed, this would be the largest storm operation in the last 10 years," he said.

"We've never seen these cyclonic winds last for 24 hours straight. That's what's caused the majority of the damage."

The storm has left local authorities across the region facing a huge clean-up task ahead of Anzac Day, with Newcastle Lord Mayor Nuatali Nelmes telling AM the bill could exceed the $1.7 billion spent in the aftermath of 2007's Pasha Bulker disaster.

The Insurance Council of Australia said it had received more than 19,500 claims so far and has declared a catastrophe in storm-hit areas.