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.
Meanwhile, vast parts of Queensland, including Brisbane and the Gold and Sunshine coasts, could face potentially life-threatening weather over the next day.
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 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.
Meanwhile, emergency services on Monday rescued 20 dogs from rising floodwaters that menaced a breeding kennel in a suburb northwest of Sydney.
Barks emanated from partially submerged kennels as the rescuers approached in inflatable boats. The dogs, a mix of Belgian Malinois and German Shepherds, were placed in animal carriers and then onto the boats, which were attached together, before being ferried over the floodwaters to safety.
"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.
'Life threatening' rain hits Queensland
The extreme weather causing havoc across southern Queensland is expected to persist for at least another day as roads are cut, schools closed and evacuations triggered.
Four homes on the Gold Coast have been evacuated due to landslide risk while more than 100 properties are being inspected for damage in the south east, Premier Annastacia Palaszczuk told parliament on Tuesday.
Ms Palaszczuk said the "rain bomb" would continue for at least another day, and emergency services had already responded to 840 calls for help.
"If you do not have to be on the roads please stay off them, we will continue to keep Queenslanders informed and safe," she said.
At least 18 schools are closed in the region, mainly because of cut roads.
The Bureau of Meteorology is warning of possible flash flooding in Brisbane, Ipswich, Gold Coast, Toowoomba, Warwick, Stanthorpe, Kingaroy, Dalby, Goondiwindi, Roma, Charleville and St George.
It says six hour rainfall totals of between 100mm and 150mm are likely to hit those areas.
"The situation is likely to pose a serious risk to areas recently affected by heavy rainfall, including the risk of landslips in steep terrain; in these areas the situation may become life-threatening," the BOM said in an alert on Tuesday.
A flood watch has been issued for the vast areas of southern Queensland between the South Australian border and the east coast.
The Nerang and Coomera rivers on the Gold Coast have burst their banks, and there's a flood warning for Maroochy and Mooloolah rivers on the Sunshine Coast.
Minor flood warnings have also been issued for the Albert and Logan rivers, south of Brisbane, and a number of rivers in the Maranoa and Warrego districts in the state's southwest.
Queensland Fire and Emergency Services coordinator Brian Cox urged people not to leave their homes and if they do to take extreme caution.
"Be really vigilant, I mean the first thing is don't go out today, don't," he told 4BC Radio.
"But if you do, please drive by the road conditions, look out for pedestrians, cyclists. We're asking people to be really really cautious.
"It will not take much with the current rainfall they've already experienced, and the saturation levels we've currently got across southeast Queensland, for any minor storm to hit, to raise those flood levels and water levels across roads and as you know that can be quite dangerous."
People living on two streets in the Gold Coast suburb of Wongawallen were evacuated on Monday amid fears of a landslide.
Floodwaters had gouged out the foundations of at least one home, leaving it precariously perched atop a slope.
Seqwater has started to release more water from the Enoggera, Gold Creek, Hinze, Lake Macdonald, Leslie Harrison, Little Nerang, Sideling Creek and Wappa dams.
The company said the Nindooinbah and Poona dams were also overflowing.
- AAP and Reuters
