The Bureau of Meteorology (BOM) has warned dangerous conditions will continue into Monday, with "life-threatening" flash flooding and damaging winds persisting overnight.

Key points: The Bureau of Meteorology has warned of dangerous flash flooding across NSW

The Bureau of Meteorology has warned of dangerous flash flooding across NSW More than 100,000 homes and businesses are without power

More than 100,000 homes and businesses are without power The State Emergency Service responded to more than 4,100 calls on Sunday

The affected areas include the Hunter, Central Coast, Sydney metropolitan area, Illawarra and eastern parts of the Central Tablelands, including the Blue Mountains.

The NSW State Emergency Service (SES) has issued two flood evacuation orders and an evacuation warning for areas at risk on Sunday night.

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Residents around Narrabeen Lagoon in Sydney's northern beaches were told to evacuate by 10:30pm.

The SES said the lagoon was rising quickly and, with a high tide expected at 10:45pm, residents may become trapped and conditions may be too dangerous for people to be rescued.

People have been advised to stay with family or friends, or head to evacuation centres at Mona Vale Memorial Hall or Cromer Community Centre.

Elsewhere, residents in Moorebank, Chipping Norton and Milperra were told to evacuate high danger areas towards Liverpool by 7:00pm.

The order is for all affected properties in Arthur Street, Rickard Road, Davy Robinson Drive and Newbridge Road between Milperra Bridge and Riverside Road.

An evacuation centre has been set up at the Velodrome in Bass Hill.

Flooding on Garden Street in North Narrabeen. ( ABC News: Dale Drinkwater )

Residents in the North Richmond Lowlands area have also been told to prepare to evacuate on Saturday evening due to rising flood levels which could cut off the North Richmond Bridge evacuation route.

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A severe thunderstorm warning has also been issued for the Central Tablelands, North West Slopes and Plains and Central West Slopes and Plains.

The SES said storms will bring heavy rainfall which could lead to flash flooding in areas including Tamworth, Gunnedah, Coonabarabran, Bendemeer, Tambar Springs and Dunedoo.

It comes as heavy rain has been falling steadily across much of the state since Wednesday, with some of the strongest falls until Sunday seen in northern NSW, near Byron Bay and Kingscliff, which had close to 300mm fall on Saturday.

Thousands were without power on Sunday afternoon as the wild weather downed powerlines.

Ausgrid said 77,000 customers were without power in Sydney, the Central Coast and the Hunter, while Endeavour Energy said 27,000 customers were without power across Western Sydney, the Blue Mountains and Illawarra.

Ausgrid said power is out for 70,000 customers. ( Twitter: Ausgrid )

Transport NSW warned there was extensive disruption to roads and public transport, including the closures of the Wakehurst Parkway at Narrabeen, the Windsor and Richmond bridges and the northbound lanes of South Dowling Street in Surry Hills.

Bus services were replacing services including the Manly and Parramatta ferries, and Blue Mountains, South Coast lines and Central Coast and Newcastle trains where lines were closed.

A fallen tree hit a car in North Narrabeen. ( ABC News: Dale Drinkwater )

Drivers and transport users are advised to check for updates.

In Sydney's CBD on Sunday afternoon, five people were treated for potential neck and spine injuries after a tree fell on a car on Bridge Street.

Four passengers in their 50s were taken to St Vincent's and Royal Prince Alfred hospitals in stable condition in what NSW Ambulance said was a "lucky escape".

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NSW Ambulance Inspector Giles Buchanan said the wet, windy weather was wreaking havoc.

"Please do not underestimate these conditions, they have the potential to be deadly," he said

The SES said it had taken 4170 calls since midnight on Saturday, 58 of which resulted in flood rescues.

Four men were rescued on Saturday night after their boat capsized in Brisbane Water around 11:30pm.

NSW Police Assistant Commissioner Karen Webb said the incident showed people were not heeding warnings about the dangerous conditions.

"That was very reckless and unnecessary, and put lives at risk," she said.

Amid the wet conditions, the NSW Rural Fire Service night said on Saturday the Currowan Fire in Shoalhaven was officially out after burning for 74 days and destroying 312 homes in towns including Lake Conjola.

It burnt through 499,621 hectares.

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BOM forecaster Jane Golding said such prolonged and heavy rain was unusual.

"It is uncommon to see rainfall rates as high as we have seen, up in the hundreds of millimetres occurring over consecutive days," she said.

The Bureau of Meteorology issued a severe weather warning extending the length of the coast, with torrential rainfall and damaging winds of up to 90 kilometres per hour expected.

The powerful winds, rain and a king tide are expected to cause erosion along parts of the coast.

Flood alerts have been issued for Byron Bay, Coffs Harbour, Port Macquarie, Taree, Newcastle, Gosford, Sydney, Katoomba, Lithgow, Wollongong, Nowra, Ulladulla, Eden and Moruya Heads.

Strong winds and a king tide caused rough conditions at Dee Why. ( Twitter: Dallas Kilponen )

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Emergency Services Minister David Elliott said people should "reflect on the need to go out today".

"What we will see today is very dangerous driving conditions so people are encouraged not to drive through roads or across bridges that they can't see," he said.

The Mid-North Coast received 250mm of rain in the past 24 hours, while 150mm fell across the Blue Mountains and Central Coast.

Ms Goulding said as the system moved south, Sydney and the Central Coast should expect to see similar rainfall totals that fell across the north of the state.

"The last time Sydney saw as much rainfall as we have had now, that was [in] 2016 … but we have surpassed those figures and you have to go back as far as 1998 to see totals like we will get."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 43 seconds 43 s Torrential rain saw surging waters in creeks across northern Sydney in Hornsby and Killarney Heights

Authorities say the weather system will help top up Sydney's water catchments.

Warragamba Dam has been at 44 per cent capacity but Water NSW said, with its catchment area set to gain 130mm of rain on Saturday, capacity could rise to 55 per cent.

It said it was set to receive its best inflows since April 2017, and could effectively recoup nine months of water supply in less than a week.

The Yarramundi Bridge, near Richmond, on Sunday morning. ( Facebook: NSW SES Hawkesbury Unit )

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Adrian Langdon from Water NSW said farmers were also cheering with some parts of the state receiving their best rainfall in almost a decade.

"For some of those agricultural areas that have been extremely dry for the last three to five years, [they're] getting some good rainfall as well."

The BOM said winds would generate large, powerful rains in combination with king tides which could lead to significant coastal erosion.

Massive waves were seen moving up a cliff face at Kurnell, creating what has been described as a "reverse waterfall".

The NSW RFS said the rain had reduced the number of fires burning across the state to 37.

Mr Elliott said he was pleased fire conditions had eased, and while there was still work to be done, he didn't expect the normal bushfire season would need to be extended.

Meanwhile, following Mr Elliott's warning against "boofhead behaviour" prompted by footage of a man riding a jet ski on a flooded road in Tuggerah on the Central Coast, more antics have emerged.

A man was filmed riding a bodyboard at a drive-through of a fast-food restaurant, in what is believed to be the same area as the jet ski footage.