Hundreds of stranded residents have been rescued from their homes and cars in NSW, as a "life-threatening" downpour that has triggered record-breaking floods heads to Sydney.

Vision of entire homes adrift in floodwater, cars inundated and people forced to evacuate their homes in tinnies have been captured in social media on the NSW Mid-North Coast, where major flooding has hit several towns.

Record-breaking rainfall has also hit parts of the mid-north coast and Hunter region, with rainfall above 100-200mm above March records. 

Residents of the mid-north coast are being warned to expect more rainfall this evening and into Sunday.

“They're not quite out of the shadows yet. And the flood risk will remain,” Agata Imielska, senior climatologist at the Bureau of Meteorology, told reporters on Saturday.

The town of Kendall has been hit by 405mm of rain, while Red Oak was pelted with 371mm.

“It's a very significant, record-breaking event with the rainfall that we have seen,” Ms Imielska said.

The lower Blue Mountains are expected to see the worst of the rain on Saturday and into Sunday.

For greater Sydney, much will depend on exactly where the rain falls and what happens when Warragamba Dam, the city's main water source, begins to overflow on Saturday afternoon and dumps more water into heaving water courses.

"Based on our current modelling, we're thinking that we might see some minor flooding at Penrith and North Richmond later today," Bureau of Meteorology national flood services manager Justin Robinson told reporters. There are also concerns for Windsor, northwest of Sydney.

He said the major river systems around Sydney were expected to see minor to moderate flooding but warned: "Major flooding is definitely a possibility with this rainfall event".

Concerns are focused on communities along the Georges River, an urban river in the city's south, and on the Hawkesbury/Nepean river system to the north and west of Sydney.

"It's a very dynamic and evolving flood situation and we could see some very deep and rapidly responding rivers with very high levels.

"We've got multiple flood warnings impacting coastal rivers all the way from the top of the mid-north coast ... all the way down into the Sydney region."

Those in low-lying properties around Taree, Dumaresq Island, Cundletown, Central Wingham and Wingham Peninsular were ordered to evacuate on Saturday morning.

Evacuation orders were also issued in Kempsey, Port Macquarie, North Haven, Dunbogan, Camden Head and Laurieton on Friday.

The Hastings and Camden Haven rivers have reached record levels. The Hastings River peaked at 12.1m near Port Macquarie breaking an eight-year-old record, and major flooding at Laurieton exceeded a record set in 1978.

The State Emergency Service has responded to more than 3200 calls for help since the wet weather began, including 335 flood rescues overnight.

That is a "huge number", NSW SES spokesman Andrew McCullough told AAP.

"A lot of those jobs are for people who are stuck in their houses, caravans, animals isolated by floodwater, people stuck in cars - it's really for a wide mix of things," he said.

Most of the calls are coming in from Taree, Wingham, Port Macquarie, Macksville, Settlement Point, Wauchope, Laurieton and North Haven.

"Every minute we get two or three more rescues coming through at the moment."

"This is not a typical sort of flood, these are life-threatening floodwaters and (it's) a very serious situation."

Calls for help from the Hunter and Sydney regions are starting to roll in too, as the system moves south, Mr McCullough said.

The Bureau of Meteorology issued a severe weather warning on Saturday morning covering an area from the mid-north coast to the far south coast of NSW as well as Canberra.

The bureau warned of intense rainfall, flash flooding, damaging winds with gusts exceeding 90km/h, dangerous surf and coastal erosion.

Rain has plagued the Sydney area for most of the week, but the deluge arriving on Saturday will be on another level, the BOM says.

"We might see quite significant flash flooding and we've got a flood watch current for both the Nepean and Hawkesbury rivers," bureau flood operations manager Justin Robinson said.

"We're likely to see a spill from Warragamba Dam and that will then impact those communities downstream."

The Public Information and Inquiry Centre has been activated, allowing members of the public to contact 1800 227 228 for information about the severe weather and flooding at any time of day.

The deluge is set to continue well into next week and could deliver the heaviest rainfall since February 2020, when Greater Sydney was hit.