NSW wild weather: Second storm cell expected to hit parts of Sydney and Central Coast.

A second storm cell developing off New South Wales is expected to hit northern parts of Sydney and the Central Coast in the next few hours.

By: ABC NEWS

Emergency information

ABC Emergency

Bureau of Meteorology NSW weather warnings

NSW State Emergency Service

NSW Government emergency information

Listen live: 1233 ABC Newcastle

Listen live: 702 ABC Sydney

For information about the storm and flood operation, call the Public Information and Inquiry Centre on 1800 227 228

For help during a storm or flood, call the SES on 132 500

In a life-threatening emergency, call 000

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”.

The Bureau of Meteorology says winds are expected to reach gale-force from the Illawarra to the Hunter region, including Sydney, later this morning. Rain could be heavier than yesterday before conditions ease later in the day.

Some 200,000 homes are still without power and almost 100 people have been rescued from floodwaters.

Emergency services have responded to more than 8,000 calls for help to address flash flooding, fallen trees and downed powerlines.

The State Government is urging people to commute outside peak hours to minimise the pressure on struggling road and public transport networks.

“We’ve seen this morning the closure of the Bankstown line due to flooding,” New South Wales Roads and Transport Minister Andrew Constance said.

“It’s obviously a very difficult situation we’re all facing at this time, and anything we can do to minimise travel we’re urging people to do so.”

Ferries are set to resume between Circular Quay and Manly today after the Harbour Master shut down operations yesterday because of rough conditions around the Heads.

The timetable today will depend on how steady those ferries can be kept at Manly Wharf.

Regarding train disruptions, the Hunter Line is closed and there are partial closures on the south coast, Central Coast and Newcastle lines.

An evacuation warning remains in place for residents around the Manly Dam and lagoon district, but so far there has been no overflow from the dam.

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.”

The State Emergency Service (SES) says extra crews have been drafted in from interstate to help volunteers in the Hunter region.

The SES's Andrew Richards said crews conducted six flood rescues around Maitland overnight, including of 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.

Fellow SES worker Todd Burns said there had been a little relief for emergency crews overnight.

"We've had a bit of a reprieve from the rain and wild conditions," he said.

"The areas worst affected have been the Sydney northern region stretching into the Central Coast and also above that, the Hunter region.

"What we'll be doing is reactivating crews that have worked hard overnight on the priority jobs and as we see more damage and more reports of damage with the aid of daylight."

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

"Cyclonic" conditions yesterday battered parts of the state wreaking havoc on the Hunter, Sydney, Central Coast and Illawarra regions.

Dungog local police commander Jeff Loy said the deaths of three elderly people in the 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.

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, very quiet and sombre".

The town of Greta was also hit hard. Resident Henry had to be rescued as his house was swallowed up by water.

"It was a bit scary, especially when I walked outside," he said.

"By this time it just rose so quickly. I was rescued by some neighbours and the water was chest high."

SES deputy commissioner Steve Pearce said the 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."

A version of this story was originally published on ABC.