Greater Sydney is on high alert with the next 24 hours "absolutely critical" with torrential rain and flash-flooding forecast, while the confirmed death toll in the NSW flood crisis continues to rise.
"Early tomorrow morning and throughout the day, we could see severe thunderstorms bringing heavy rain, damaging winds and possibly even large hail," the Bureau of Meteorology's Dean Narramore said on Wednesday.
The severe weather warning extends from Newcastle to the South Coast and there's a flood alert for the Hawkesbury and Nepean rivers with residents warned they may need to evacuate.
Speaking from flood-affected Grafton, Premier Dominic Perrottet said the unprecedented flood crisis unfolding from the north to the south of the state required an unprecedented response.
"Saving human lives is our number one focus," he said.
"Our commitment as a government is to get every single person through this."
The government would "not spare a dollar" in getting flood stricken communities back on their feet.
Emergency Services Minister Steph Cooke said "Sydney is on high alert".
She urged people to comply with evacuation warnings and orders, saying "the next 24 hours are absolutely critical".
"While we're all hoping for the very best, we must be prepared for the worst," she said.
'"Sydney is copping a battering ... that will continue for some time."
The focus for Sydneysiders remains on Warragamba Dam which is overflowing, threatening up to 130,000 homes in the Hawkesbury and Nepean area.
"All of our attention is on Warragamba Dam ... and at this stage, we are expecting something similar to March 2021," Ms Cooke said.
SES Commissioner Carlene York warned communities in the Hawkesbury-Nepean they must be prepared for the severe flooding.
"We are expecting this might be as bad as we saw last year," she said.
Meanwhile, the death toll from flooding in Lismore has risen to four.
The body of a man in his 70s was found in a unit about 2pm, hours after another man's body was found floating in flood waters on a Lismore street on Wednesday morning.
That grim discovery came after the body of a woman in her 80s was found inside a South Lismore house on Tuesday afternoon.
She was the second elderly woman to die after being trapped in a flooded home in the area.
Another man who disappeared in floodwaters in Lismore on Sunday is presumed dead.
A fifth man died on the Central Coast on Friday when his car was swept away by floodwaters.
The SES warns heavy rain and strong winds are starting to impact the Central Coast, Sydney, Illawarra and South Coast areas.
"The ground is waterlogged, and with the strong winds, there's a real risk of falling trees," it noted.
The heavy rainfall will also affect Newcastle, the Blue Mountains and Wollongong.
NSW SES is advising residents within parts of Chipping Norton and Lansvale in southwest Sydney to evacuate in the next few hours.
The SES received more than 8800 calls for help overnight on Tuesday and undertook 300 flood rescues in the northern area of the state which has been inundated with record floods centred on Lismore.
There are several major flood warnings for the Greater Sydney including Hawkesbury River, Nepean River, Camden, Wallacia, Penrith and Windsor.
The slow-moving weather system arrived in Sydney late on Tuesday, dumping rain over parts of the Hunter and Metropolitan, Illawarra, South Coast and parts of Central Tablelands and Southern Tablelands.
"It is going to be a mammoth task," Mr Perrottet said of the clean-up.
