Hundreds of millimetres of rain have drenched the NSW coast, sparking warnings of high winds, "life-threatening" flooding and landslips as the state faces its most torrential downpour since 1998.

Some areas across the state recorded more than 300mm of rainfall within 48 hours, with Bureau of Meteorology acting NSW state manager Jane Golding warning the situation would only intensify on Sunday.

A severe weather warning for very heavy rain, high winds, large waves and tides has been issued along the entire coast from the Northern Rivers region to the South Coast.

"Potentially we haven't seen anything like this since the late 1990s," Ms Golding said.

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Speaking at a press conference on Saturday afternoon, Ms Golding said NSW was facing a "very dangerous system".

"We've been given a bit of breathing space but it's just starting to increase now, this is the most intense development of the system."

She warned of significant overland flooding, flash flooding and landslips, adding that the recent bushfires in some areas have made landslips more likely.

BOM senior forecaster Jordan Notara on Saturday afternoon said they could see 100 to 200mm falling over metropolitan Sydney and surrounds within the following 12 to 24 hours.

He warned that could cause "inundation on some places along the coast, minor to major flooding over some catchments and very strong winds along the coastline".

Cudgera Creek, in NSW's north east, recorded 320mm of rain in the period between 9am on Saturday and 4am on Sunday.

In the same period Tweed Heads recorded 235mm, Comboyne Public School recorded 189mm, Wentworth Falls recorded 155mm, and Ballina recorded 140mm.

NARROW ESCAPE AS 4WD SINKS IN 15 SECONDS

Footage has emerged showing the frantic moment a woman and her niece made a last second escape from their sinking 4WD after being swept off the road by floodwaters in northern NSW.

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The 67-year-old woman told Nine News it took all of 15 seconds for her and her niece to realise they were in trouble after becoming trapped in floodwaters near Lismore.

The two women scrambled out of the windows of the Mistubishi Pajero after being washed off the road by floodwaters. Photo / Supplied
The two women scrambled out of the windows of the Mistubishi Pajero after being washed off the road by floodwaters. Photo / Supplied

"Oh my God, holy crap," a woman filming in a trailing vehicle can be heard saying in the footage.

"What are we going to do?" her father can be heard asking as the car stops at the edge of the floodwaters.

"Oh my God, their car is literally rolling off. They need to get off. Their car is sliding, they need to get out," she says as her father wades in to help.

"It's gone! It's literally gone," the woman continues in commentary as the pair begin walking away from their submerged 4WD.

News.com.au understands the women were driving from Byron Bay to Lismore and the road wasn't closed when the incident occurred but has since been shut by the local council.

The NSW SES has received more than 2600 calls for help since midnight on Wednesday and responded to at least 40 flood rescues, mostly involving motorists who drove into floodwaters, with six flood rescues alone performed in the Grafton area overnight.

The NSW SES are being assisted by police.

"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," said Deputy State Emergency Operations Controller, assistant commissioner Karen Webb.

Assistant commissioner Webb added that police have also responded to or been made aware of incidents resulting from people taking unnecessary risks or displaying dangerous behaviour.

"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," she said.

Officers from Brisbane Water Police District and local residents put their lives at risk on Saturday night to rescue five men whose fishing boat capsized as they tried to navigate in the dark to go fishing.

"Thanks to the courageous actions of rescuers, the men were safely returned to shore before being treated for possible hypothermia and cuts and abrasions," assistant commissioner Webb said, adding that none of the men were wearing a life jacket and had struggled to swim.

A 16-year-old boy was also rescued by the SES after becoming trapped between debris in waist-deep water after falling into the Allynbrook River on Sunday morning.

The teenager, who has been taken to hospital with suspected broken ribs, was in the water for almost two hours before specialist officers were able to free him.

One family was rescued by the Rural Fire Service via helicopter after flood waters cut them off, leaving them isolated in the car, an SES spokesman told AAP on Sunday.

The SES on Saturday night urged residents across the state, including Sydney and the South Coast, to prepare for heavy rain and potential flooding as a coastal trough causing rain and windy conditions continues moving south throughout the rest of the weekend.

NSW SES commissioner Carlene York urged those in low-lying areas to plan ahead.

"Now is the time to plan how you will protect your family and property from flood," Ms York said in a statement.

"Never drive, ride or walk through floodwaters. It's too dangerous – and you never know how deep the floodwaters are, the condition of the road or how fast the water is flowing.

"Please keep clear of drains, creeks and causeways and if you are in a flood-prone area avoid parking vehicles in low-lying areas."

The BOM has issued minor flood warnings for the Bellinger River, Hastings River, Georges River, Tuggerah Lake, Paroo River, Weir River and the Cooks River.

A minor to moderate flood warning is also in place for the Manning and Gloucester rivers, the Hawkesbury, Nepean and Colo rivers, the Orara River, and a moderate flood warning for the Macintyre River.

An initial minor flood warning is in place for the Tweed River, Wallis Lake and the Camden Haven River.

Major flooding is likely in the Colo River and Upper Nepean River with minor to major flooding in the Lower Nepean and Hawkesbury.

Flooding in these regions is particularly dangerous due to the "Bathtub Effect" caused by choke points where the rivers narrow, waters back up and begin spilling over the riverbanks.

The NSW SES said they had pre-positioned resources in regions which were expected to be flooded and warned residents to avoid unnecessary travel during storms.

'A VERY DANGEROUS SYSTEM'

"We are seeing the signs that an east coast low will develop," Ms Golding said on Saturday afternoon, which would bring gale force winds to some parts of the coast.

A King tide is expected on Monday morning which will increase the risk of flooding.

The NSW State Emergency Services will be assisted by police.

NSW Police assistant commissioner Karen Webb repeated warnings that people shouldn't drive through floodwaters.

"Because if they do and become stuck people get trapped, and that's a drain on resources."

She also advised against anyone taking a boat out in the current conditions, which are already not suitable and expected to get worse.

The SES issued a fresh warning to motorists after rescuers came to the aid of 13 cars which became stuck trying to drive through floodwaters on the NSW Central Coast.

"There's always an alternative route – never drive through floodwaters," NSW SES spokesman Terri Langenemdam said, after the 13 carloads of people had to be rescued in the Gosford and Wyong areas on Friday night.

"People think that it looks OK on the surface but there can be debris underneath, roads can be washed away."

"There's also a reminder to keep kids, away from creeks, drains and causeways."