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Thousands More Face NSW Flood Evacuation As 'Catastrophic' Weather Continues

Some 18,000 NSW residents have already been evacuated from their homes to escape rising floodwaters, as the premier admits conditions are set to deteriorate and the fallout will linger for weeks.

Another 15,000 residents are on high alert after being warned they must be ready to leave on Tuesday as two "catastrophic" weather fronts roll across large swathes of the state.

People northwest of Sydney have been ordered to evacuate homes amid the unrelenting downpour as a massive swathe of water flows into catchments, causing rivers to rise and break their banks.

Major flooding is occurring along the Colo River in the Hawkesbury region and the State Emergency Service ordered about 500 people in 200 homes to get out on Tuesday.

SES Commissioner Carlene York on Tuesday told reporters those people would otherwise have been trapped by the rising waters.

Boats and helicopters would be deployed to help them leave.

On the Hawkesbury River, another evacuation order was issued for low-lying properties in North Richmond, Agnes Banks and Windsor.

Warnings of moderate flooding along the Nepean River at Penrith are in place and floodwaters are expected to affect the upper Nepean.

Warnings are also in place for the Hawkesbury River from Windsor to Wisemans Ferry.

Relentless rain has lashed the NSW coast for days, causing major flooding similar to the 1988 and 1990 flood events along the Hawkesbury River and the Bureau of Meteorology is forecasting more heavy rain.

More than 9500 requests for help have been made around NSW since Thursday, with emergency services performing about 850 flood rescues.

Premier Gladys Berejiklian warned the weather would not ease up until Wednesday but the flood risk would last for weeks as catchments flow into already full dams and rivers.

"Please continue to be patient because even when the rain stops, the rivers will keep rising," Ms Berejiklian told reporters.

"There are several weather fronts impacting a huge area of the state.

"(It's) catastrophic in its dimensions ... but we're up for the challenge."

Two Australian Defence Force aircraft have been made available to the SES on the NSW South Coast for search and rescue activities.

Prime Minister Scott Morrison told reporters "floodwaters will persist for some time" and the government had received a request for 1000 ADF personnel to help with the NSW clean up.

"We want it to be done swiftly and effectively to try and get these communities back on their feet as quickly as we possibly can," Mr Morrison said.

No lives to this point have been lost, thankfully.

Mr Morrison said some $3.5 million had already been distributed in federal government assistance to those affected by floods.

Ms York said the rising Colo and Hawkesbury rivers concerned her most, but an inland weather system coming across from the Northern Territory would also blight rural communities such as Grafton and Lismore.

Evacuation warnings remain in place at Kempsey on the state's Mid-North coast but the Macleay River levee held overnight.

The Hunter, the Central Tablelands and the South Coast are also in for a drenching.

Major flooding is occurring at Wollombi in the Hunter Valley, while moderate flooding is plaguing Taree and Gloucester.

Parts of NSW will be hit by wind gusts of up to 90km/h, prompting fears trees could be uprooted from the saturated soil.

Weather forecasters say the record-breaking floods are amongst the worst they've seen - and there's more torrential rain to come.

The BOM is predicting rainfall of more than 100mm across the NSW south coast on Tuesday and up to 300mm in some parts.

Sydney and the mid-north coast could cop another 100mm in the next day or so, and a season's worth of rain is possible in the west.

"We have come off the back of drought and bushfires in the pandemic and other floods. Some of the same areas that were smashed by bushfires only 15 months ago are now dealing with floods," Deputy Premier John Barilaro told the Seven Network.