New South Wales authorities say they are dealing with the largest storm operation in a decade after three people were killed in "cyclonic" conditions that have battered parts of the state for hours.
Residents in the Hunter region were told to brace for more flash flooding overnight, while authorities door-knocked some Sydney residents to warn they may need to evacuate.
Conditions described as "cyclonic" have wrought havoc on the Hunter, Sydney, Central Coast and Illawarra regions, with rescue crews called to more than 1,000 storm-related incidents and more than 200,000 properties losing power.
In the Sydney suburbs of Manly and North Manly, the State Emergency Service (SES) visited homes and sent text messages to warn homes could be flooded if the Manly Dam continued to rise.
"The worst-case scenario, if the houses in our flood planning get affected, would be 660 residences," Samantha Colwell from the SES said.
"In saying that, a lot of them are apartments so obviously the people on the higher levels are not going to get inundated.
"At the moment, people can stay in their homes, but we do actually encourage them to prepare because the last thing people want to do is find out about it in the middle of the night."
An evacuation centre has been set up at the Hardbord Diggers club.
Further north, authorities warned some low-lying parts of Narrabeen and North Narrabeen, near Narrabeen Lagoon, could also be at risk, with an evacuation centre opened at Pittwater RSL and Mona Vale.
On Tuesday morning, three elderly residents were found dead in the town of Dungog, north of Newcastle, where more than 300 millimetres of rain fell in less than 24 hours.
Authorities said the circumstances around the deaths were still still being investigated.
Locals said several homes were washed away, and a woman and two children were rescued from a house as it was washed down a street in nearby Greta.
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.
"We've had over 6,500 requests for assistance and on top of that, with the enormous amount of rainfall - up to 320 millimetres in over 24 hours - we've seen about 80 flood rescues. We've seen homes washed away, whole streets decimated."
He said thousands of emergency services were on the ground and more were coming in from around the state to help on Wednesday.
The SES sent emergency alerts to more than 100,000 mobile phones in the Hunter as it was battered by relentless wind and rain on Tuesday afternoon.
The text messages warned of "rapid rises and high velocity flash flood water in local creeks, watercourses and urban areas" in Newcastle and surrounding areas.
Newcastle Mayor Nuatali Nelmes told 7.30 the city looked like it had been in a disaster movie.
She said roads were under water, trees had been blown down and roofs had been ripped from buildings.
"We're actually bracing for worse to come," she said.
"Overnight, we are bracing for potentially more flooding in Newcastle and the Hunter ... so people are being urged to stay at home and to stay on high ground where it's safe."
In the eight hours to 5:00pm Tuesday, Maitland received 274 millimetres of rain, Seaham received 152 millimetres and Tocal received 137 millimetres.
The Bureau of Meteorology (BoM) issued a severe weather warning for damaging winds and surf and heavy rain along the coast from the Illawarra region to the Hunter.
It said an intense low pressure system was centred just off the Hunter coast, near Newcastle.
"This low is expected to remain slow-moving overnight, maintaining vigorous winds, large seas, and periods of heavy rain and thunderstorms," BoM said in a warning issued just after 11:00pm.
"Conditions are expected to slowly ease during Wednesday as the low weakens."
An evacuation centre was set up at Dungog High School following reports at least 20 homes had been inundated, but it has since been closed.
However, evacuation centres have been opened at the Senior Citizens Centre in East Maitland and the Shamrock Multipurpose Centre at Ashton Field.
NSW Police said residents who evacuated their homes should take important documents and photos, spare clothing and medication.
Those using the evacuation centres were asked to bring blankets or sleeping bags, pillows and sleeping mats.
Local police commander Jeff Loy said the three deaths occurred in different locations in Dungog.
"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.
One talkback caller, David, told 1233 ABC Newcastle he was in the town visiting family but
got out before it was cut off on Tuesday afternoon.
"I expect there is 50 to 60 people whose houses have flooded," he said.
"There were people sitting on their roofs.
"[Some people] have nothing left - they don't have a wallet, they don't have anything. They got out within minutes, these people.
"Water's up to their ceilings, people were swimming to try and get up on their roof.
"There are animals floating around all over the place."
He said the local timber mill had been "smashed", which would have lasting repercussions for the local economy.
Premier Mike Baird said the loss of three residents was a tragedy.
"It's hard to imagine that they were just going about their lives a short time ago," Mr Baird told 1233 ABC Newcastle.
"Our thoughts and prayers are with their family and friends."
Earlier, he encouraged people to leave work early to avoid being caught in the worsening weather.
"We are calling for bosses to be flexible, people to make arrangements in an orderly way to start to head home as soon as you possibly can."
Dozens of schools in the Hunter area have been closed until at least Thursday.
The weather caused major transport disruptions, including cancellations and delays to train, bus and ferry services.
Transport authorities urged people to avoid all non-essential travel, both by car and public transport, and check timetables for updated information.
More than 100 sets of traffic lights were blacked out and some major road networks were affected by flooding.
Sydney Airport advised passengers to check with airlines for information about delayed and cancelled flights.
Electricity distributor Ausgrid said there were more than 4,500 reports of hazards, such as fallen wires, across its network.
Crews were responding to thousands of urgent incidents and restoring power to more than 200,000 properties would take several days, he said.
"For those customers that are in more inaccessible areas [it will take until] towards the end of the week or even more," he said.
"The problem is in areas, for example, around Maitland where we are being advised we haven't seen the peak of floodwaters, so crews aren't able to get into the area to make their assessment."
The organisation has deployed extra resources to Newcastle and the Hunter area.