Floodwaters have isolated thousands of NSW residents and caused evacuations in the state's north, as ex-tropical cyclone Oswald beat a devastating path towards Sydney.
About 2000 people were cut off in the state's drenched north yesterday afternoon, with about 850 people isolated at Ewingar, west of Lismore, and 400 at Darkwood, on the Bellinger River.
At 4.30pm, flood warnings were in place for eight NSW river systems, the Bureau of Meteorology said on its website.
In Lismore, the NSW State Emergency Service (SES) was doorknocking homes and 500 residents were being evacuated, with the Wilsons River at a height of 9.7 metres.
Meanwhile, in Grafton, more than 1500 people are expected to be evacuated today as the Clarence River continues to rise.
At Bellingen, floods cut the town in half after a main bridge, Lavenders Bridge, went under water, with the Bellinger River tipped to reach 6.5 metres.
The Bureau of Meteorology has warned residents near the Tweed River, Wilsons and Richmond rivers and the Clarence Valley that they could face major flooding. Flood warnings are also in place for the Bellinger, Macleay, Hastings, Brunswick and Nambucca rivers.
By 2pm yesterday, the SES said it had received 1400 calls for help, mainly from the impacted north-east corner of NSW.
SES spokesman Phil Campbell said there had been 13 flood rescues, including several after people ignored warnings and tried to drive through floodwaters in the Tweed area.
In Sydney, flood rescue crews were on standby in the north of the city, where the SES expected storm, coastal erosion and rescue operations to ramp up over the next 24 hours.
A severe weather warning takes in major centres including Sydney and the Hunter Region, with the weather system expected to reach the Illawarra at dawn today.
The bureau forecast that Sydney and Newcastle would probably have started seeing the worst of it by last night, with winds forecast to reach up to 100km/h and downpours of up to 200 millimetres.
Northern Sydney SES incident controller Tony Pinelli said rescuers were bracing for prolonged rainfall, gale-force winds and damaging surf conditions.
The SES said waterfront residents were sandbagging yesterday.
Flood rescue specialists and boats, as well as two winch-capable helicopters, were ready.
The Ambulance Service has deployed 11 paramedics trained in swift-water rescue to Coffs Harbour.
Authorities also warned those travelling home after the long weekend to be extremely careful, with many roads impacted by flash flooding. AAP