Firefighters say the past week's torrential rain has extinguished the Gospers Mountain "mega-blaze" north-west of Sydney and is on track to put out the state's remaining fires this week.

Key points: About 90,000 homes remain without power in NSW

About 90,000 homes remain without power in NSW Evacuation orders are in place along the Georges and Hawkesbury rivers

Evacuation orders are in place along the Georges and Hawkesbury rivers The Insurance Council of Australia has declared the rain event a "catastrophe"

The fire, which burned through more than 512,000 hectares after it was ignited by lightning strikes in a remote forested area on October 26, was once considered "too big to put out".

The Bureau of Meteorology (BOM) said the past four days in Sydney had been the wettest since 1990, with a combined 391.6 millimetres of rain falling.

The torrential downpour has thrown the city and Sydney's outer suburbs into disarray, with flash flooding, evacuation orders, trees toppling onto cars and disrupted train services.

Around 200 people have so far been rescued after driving into floodwaters.

A bus driver and six children had to be rescued from floodwaters in Maraylya. ( Facebook: NSW SES Hawkesbury Unit )

More than 15,600 calls — a record number — were made to triple-0 in the 42 hours from 5:00pm on Saturday, with Fire and Rescue NSW attending 3,068 incidents.

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Fire and Rescue NSW Deputy Commissioner Jeremy Fewtell said a significant number of those were "genuinely life threatening".

"The number of calls that we received in the peak period was actually almost double that we were receiving at the worst of the bushfires," he said.

"So I think that gives a good indication of the scale of the impact on such a broad part of the community of NSW."

Earlier, NSW Police concluded their search for a driver feared missing in floods after his car was washed off a causeway at Galston, in the city's north.

The State Emergency Service (SES) and police divers were unable to locate a vehicle but said they would continue conducting inquiries about the incident.

Despite the chaos, the end of the Gospers Mountain fire represented a major turning point in this season's unprecedented bushfire season.

The rains also put out the Erskine Creek, Ruined Castle, Green Wattle Creek and Currowan fires.

There are now just 17 fires burning across the state.

"Firefighters and obviously supporting emergency services personnel are absolutely ecstatic," said NSW Rural Fire Service (RFS) spokesman James Morris.

The fire which damaged and destroyed homes across Lithgow, the Hawkesbury and the Central Coast, was officially declared "out" at 8:20am.

"A very damaging and destructible fire … a number of those communities obviously rejoicing with this rain that they've had now, and will now start that process of rebuilding."

This tree uprooted part of the street when it fell on this car in Sydney's northern beaches. ( Twitter: Nott_Richard )

Mr Morris said five fires remained uncontained across the state, all within the Bega and Snowy Valley areas.

He said the RFS was on track to have all the remaining fires in the state put out some time this week.

For today, the clean-up continues, and the SES urged people in Sydney to stay home from work after dozens of schools were closed and several evacuation orders were put in place

"Everywhere has been hit, it's hard to pinpoint where it's worst," Matt Kirby from the SES said.

The rain has, however, meant the Warragamba Dam — which supplies 80 per cent of Sydney's water — could soon fill to 70 per cent capacity.

Just days ago it was at 42 per cent.

"This is going to make a considerable difference to Sydney's water situation," Water NSW spokesperson Tony Webber said.

The Insurance Council of Australia has already declared the weekend a "catastrophe", with around 10,000 claims lodged yesterday.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 24 seconds 24 s The Parramatta River overflows as floods grip Sydney.

The Hawkesbury-Nepean, Georges, Parramatta and Colo rivers have all spilled over and caused localised flooding.

Evacuation orders were issued for suburbs including Moorebank, Chipping Norton and Milperra as the Georges River swelled to unprecedented levels.

Last night residents at Narrabeen Lagoon, on Sydney's Northern Beaches, fled for higher ground as their waterfront properties came under threat.

As the system moved to the South Coast, people in Lake Conjola — where the bushfire clean-up continues — were warned flooding is likely.

Floodwaters have also risen around Lake Illawarra and road access may be cut.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 37 seconds 37 s Floods cause havoc across Sydney

Mr Kirby said people should work from home today if they could, and that the organisation was expecting many more calls for assistance.

A total of 54 schools have been closed in areas including Narrabeen, Picton, Gosford, Wyong, Penrith and Paramatta.

"We are trying to ask people to reconsider their travel, a number of roads are closed," Mr Kirby said.

"And we are still seeing people driving into floodwater."

The SES conducts a flood rescue in St Mary's in Sydney's west. ( ABC News )

A landslide in Leura caused a tree to topple on overhead wiring at Blackheath. ( Twitter: TrainLinkWest )

NSW Emergency Services Minister David Elliott said it was disappointing motorists were not heeding warnings after he called out "boofhead behaviour" yesterday.

"We have 400 SES volunteers trying to complete the backlog this morning and unfortunately this has included 150 flood rescues," he said.

"The message to the motorists of Sydney and indeed the wider metropolitan area is if you can avoid being on the roads do so."

A car crushed by a downed power pole at Coogee Oval. ( Twitter: @NancGlenn )

The Georges River is close to the top of the Milperra bridge. ( ABC News: Antonette Collins )

It wasn't just the roads being hit — the city's train transport network was also experiencing significant delays this morning.

The T1 North Shore line was out of action from Gordon to North Sydney due to a landslide at Artarmon.

A landslide at Leura in the Blue Mountains has caused trains between Springwood and Bathurst to be cancelled for several days.

There are also delays on the T7 Olympic Park, South Coast, Central Coast and Newcastle, Blue Mountains and Southern Highlands lines.

The BOM said the trough had moved away from Sydney, with heavy rain and flash flooding expected on the South Coast before the system weakens and crosses into Victoria this afternoon.

However tropical moisture will continue to circulate over NSW and thunderstorms are forecast for several areas later this week.

