Around 500,000 people across NSW are subject to an evacuation order or warning as the flood crisis moves south, with the premier saying the state is facing "a battle on two fronts".
Emergency Services Minister Steph Cooke said Greater Sydney, the Hunter and the Central Coast are all facing "treacherous weather conditions", with the next 24 hours being critical.
Major flooding is expected along the Hawkesbury and Nepean and Georges rivers, Colo and Macdonald rivers at Menangle, North Richmond, Penrith and Windsor and dozens of suburbs are on high alert.
Meanwhile, recovery is underway in the state's north, after the regional city of Lismore endured its worst-ever flood crisis.
The State Emergency Service has issued 76 evacuation orders affecting 200,000 people and 18 evacuation orders to nearly 300,000 people.
Speaking on Thursday morning, NSW Premier Dominic Perrottet said authorities expect conditions - particularly in the Hawkesbury region - will be worse than flooding events last March.
"We have half a million across our state who are subject to one of those warnings or one of those orders. If you are subject to one of those warnings, please get ready. Please ensure you are ready to evacuate. If you are subject to one of those orders, please get out," he said.
"Those instructions are not there for the sake of it, they are there to keep you and your family safe.
"In addition to that, we do believe that things will get worse before they get better here in our state."
Ms Cooke said she is worried about the communities across south-west Sydney, Sydney more generally, the Central Coast and the Hunter.
"I know you’re tired. It has been a long night. Many of you have had to evacuate from your homes over this time. It has been a sleepless night for many people," she said.
"We know that you are weary, but we need you to stay with us."
Tens of thousands of people living in Sydney's west, northwest and southwestern suburbs including Windsor, North Richmond, Camden and Milperra, have been ordered to flee to safety with evacuation centres set up in suburbs including Canley Vale and Blacktown.
Onlookers watch on as the Windsor Bridge is impacted by floodwater from the Hawkesbury river at Windsor, north west of Sydney, Thursday, 3 March, 2022. Source: AAP / DAN HIMBRECHTS/AAPIMAGE
The Bureau of Meteorology's Dean Narramore said there is still major to moderate flooding ongoing on rivers in the north of the state.
But the major concern is from the Central Coast, Hunter and into the Sydney metropolitan area.
"The east coast low last night, this morning, started tracking westwards, [it’s set] to come further south towards the metropolitan area," Mr Narramore told reporters, saying widespread heavy rainfall is expected later on Thursday and into the night.
"We are expecting severe weather. Warnings are current for heavy rainfall and damaging winds, particularly on coastal locations where it could be enough to bring down trees and power lines," Mr Narramore said.
He said between 50 and 250mm of rainfall is expected on Thursday, with isolated showers of about 200 mm in some areas that could lead to dangerous and life-threatening flooding.
'A battle on two fronts'
But Ms Cooke said the devastation is still unfolding in northern NSW, where the city of Lismore has endured its worst-ever floods crisis in recent days.
She said an additional 400 personnel will be deployed to the Northern Rivers region to assist in recovery efforts.
"We must not, and we will not take our eye off the people of the Northern Rivers region, that north-east part of our state that has been so deeply impacted by this weather event over the past few days," she said.
Flooded scenes on Ballina Road in Lismore , NSW, Wednesday 2 March, 2022. Source: AAP / JASON O’BRIEN/AAPIMAGE
"To that end, we are sending in additional fire and rescue strike teams, Australian Defence Force (ADF) personnel, Rural Fire Service (RFS) volunteers and workers."
Mr Perrottet said the state is facing the challenge of "a battle on two fronts".
Meanwhile, the SES has also put out a blanket road weather alert for all Sydney suburbs.
Endeavour Energy is advising customers in low-lying areas to prepare now for power cuts ahead of rising floodwater levels.
More than 300 properties - mainly along the Hawkesbury River below Windsor - have had power cut.
NSW Health says people isolating due to COVID-19 who need to evacuate their home because of rising floodwaters must leave.
"If you are told to evacuate, you must evacuate," acting Chief Health Officer Marianne Gale said.
"Under no circumstances should you remain in self-isolation at your residence - your safety is our highest concern and an emergency evacuation is a valid reason to leave your home."
NSW Health advised COVID-19 positive people at flood evacuation centres to notify staff, wear a mask and physically distance from others.
In Lismore on the north coast, four fatalities have been confirmed in the region's flood event so far.
Two women in their 80s and a man in his 70s were found dead in their flooded homes, while another man's body was also found.
A fifth man died on the Central Coast last Friday morning after his car was swept away in floodwaters.