NSW's coast has copped "extraordinarily" high amounts of rainfall with Sydney recording its heaviest drenching in up to three decades – but the deluge is not yet over, with forecasters warning there's more to come.

The rain was coupled with strong, gale force winds which battered the east coast since Wednesday resulting in fallen trees and powerful waves.

NSW Bureau of Meteorology acting manager Jane Golding said the weather and its impact across the state imitated an east coast low.

"Over the last week we have seen some extraordinarily high rainfall all along the NSW coast," she told reporters in Sydney on Monday.

.@nswpolice divers resume the search for a man and vehicle reportedly swept off a causeway near Galston north of Sydney #nswfloods@SBSNewspic.twitter.com/UQm0RbEbnO — Naveen Razik (@naveenjrazik) February 10, 2020

Some areas copped up to 700mm of rainfall since Wednesday including Robertson in the Southern Highlands, which saw 698mm of rain - 500mm of which across the weekend alone.

Pottsville in the northern rivers region was battered with close to 605mm of rain, while the central tablelands copped 631mm since Wednesday.

Possible missing person

Four people were taken to hospital on Sunday afternoon after a tree fell on their car in the Sydney CBD, and a 16-year-old boy was treated for broken ribs after being trapped between debris in the Hunter's Allyn River.

A car was also reportedly swept away in Galston on Sunday afternoon, but police have no evidence anyone was inside the vehicle.

A NSW Police Force spokesperson said the search was kicked off after a witness earlier reported a silver Mercedes being swept away on Sallaway Road between 4.30pm and 5pm on Sunday.

Police and State Emergency Service crews scoured the area and police divers were also dispatched.

A follow up search on Monday morning failed to find any sign of the vehicle and the search was concluded.

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Flooding continues

There was relief on Monday when floodwaters along major rivers appeared to peak overnight. However moderate flooding has persisted at North Richmond and Windsor along the Hawkesbury River, as well as Menangle on the Nepean.

Water from the Georges River was also inundating Milperra and Liverpool in western Sydney early on Monday, the NSW State Emergency Service said.

River levels at Milperra are now higher than during the 1988 flood and major flooding is impacting other areas such as Warwick Farm and Chipping Norton.

Residents near the Narrabeen Lagoon in northern Sydney evacuated overnight after warnings of significant flooding.

SEVERE THUNDERSTORM WARNING: For the North West Slopes & Plains, Northern Tablelands and parts of Mid North Coast, Hunter, Central Tablelands, Central West Slopes and Plains, South West Slopes and Riverina. Warning: https://t.co/pVHmcNfQfZ SES StormSafe: https://t.co/GLbnoZeQa7 pic.twitter.com/Ws4zUO2fMg — Bureau of Meteorology, New South Wales (@BOM_NSW) February 10, 2020

The rain has impacted parts of drought-stricken NSW including the northwest slopes, where up to 100mm of rain fell in the past few days.

"That's more rain than they have seen in quite a few years," Ms Golding said.

While the rain has been welcomed by many drought-affected communities, Ms Golding said follow-up rain was needed to make a difference.

The Central Coast, Sydney and the Blue Mountains were also soaked after receiving between 200mm and 400mm of rain from 9am Friday to 5pm Sunday.

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Ms Golding said Sydney experienced its highest multi-day rainfall figures since February 1990.

Strong winds also wreaked havoc on the coast where waves peaked at up to 10m and high tides caused extensive flooding and erosion.

The BoM has forecast more severe weather conditions on Monday with heavy rains, strong winds and damaging surf possible along the entirety of the NSW coast and inland southern NSW.

Utility companies are rushing to restore power in swamped regions, with more than 110,000 Ausgrid customers - primarily in northern Sydney and the NSW Central Coast - without power on Monday morning.

"The strong rains and wind have caused more than 3000 hazards to the electricity network such as fallen power poles, large trees across roads, damaged wires and extensive flash flooding," Ausgrid said in a statement.

Endeavour Energy has more than 19,000 customers without power.

SES call outs, travel disruption

The NSW SES has responded to about 10,000 calls for help and carried out multiple rescues of people trapped by rising rivers and floodwaters.

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"But we've surpassed those figures and you have to go back as far back as 1998 to see totals like we'll get," a weather bureau spokeswoman said.

The weather has also caused major disruptions to Sydney's public transport system and road networks, with commuters told to check timetables and travel apps before setting out on Monday morning.

Max wave runup getting quite close to homes on Narrabeen, accompanied by a bit of foam! #SydneyStorm pic.twitter.com/qwlqys4pAr — Chris Leaman (@chrisleaman_) February 9, 2020

With power outages continuing, parents are advised to check if their child’s school is open tomorrow before sending them. Many schools may still be impacted by power outages and flash flooding. #SydneyStorm pic.twitter.com/QH5aqnZIuO — Ausgrid (@Ausgrid) February 9, 2020

Some train services and harbour ferry services have been cancelled.

Sydney's dams are filling up with Warragamba dam now at more than 60 per cent capacity after jumping from around 45 per cent last week.

Power is out to more than 110,000 customers in Sydney, Newcastle & Central Coast. We’ve restored power where we can, but the storm has continued to move and damage more of our network.

Crews are working through the night to make these dangerous hazards safe. #SydneyStorm pic.twitter.com/vxWSLXK7jw — Ausgrid (@Ausgrid) February 9, 2020

Just after 1:30pm today NSW Ambulance were called to reports of a tree that had fallen on a car in the city. Paramedics treated 4 passengers in the car for neck pain and spinal precautions and transported them to hospital in stable conditions. #nswrain pic.twitter.com/WcgKqg0AOx — NSW Ambulance (@NSWAmbulance) February 9, 2020

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NSW police Assistant Commissioner Karen Webb urged drivers to take caution and avoid floodwaters.

"There are currently hundreds of calls for assistance, including trees, boulders or power poles down onto cars and homes, and across roads, as well as power outages and localised flooding impacting various roads and traffic lights," she said.

"I'm disappointed that I need to remind people to act responsibly and not to take risks in these types of conditions, especially when around floodwaters."