Emergency Services Minister Steph Cooke says Greater Sydney, the Hunter and the Central Coast are all facing "treacherous weather conditions", with the next 24 hours critical.
The Bureau of Meteorology says rainfall levels could hit a peak of 250mm on Thursday and is warning of life-threatening flash flooding and damaging winds with peak gusts in excess of 90 km/h.
The South Coast is also subject to a minor flood warning for the Shoalhaven and Sussex Inlet.
Major flooding is expected along the Hawkesbury and Nepean and Georges rivers, at Menangle, North Richmond, Penrith and Windsor on Thursday.
Premier Dominic Perrottet said 500,000 people across the state are now subject to evacuation orders or warnings.
A combined 94 orders and warnings were issued overnight.
The State Emergency Service has issued 76 evacuation orders affecting 200,000 people and 18 evacuation warnings have been sent to nearly 300,000 people.
"If you are subject to one of those evacuation warnings, please get ready...If you are subject to one of those evacuation orders, please get out," Mr Perrottet said on Thursday.
"Those instructions are not there for the sake of it, they are there to keep you and your family safe.
"We do believe that things will get worse before they get better here in our state," he said.
"But my message to you is that we will get through this."
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.
SES assistant Commissioner Sean Keans says a looming east coast low off Newcastle will bring heavy rain to the Hunter, the Central Coast and Sydney, the Illawarra and the South Coast from Thursday morning.
River levels are rising extremely fast due to the sheer volume of rain, with flood levels along the Hawkesbury-Nepean set to be worse than those which affected the area in March 2021, Keans said.
The Nepean River at Penrith will reach nine metres bringing moderate flooding, the Hawkesbury River at North Richmond will reach 15 metres bringing major flooding.
At Windsor, the Hawkesbury River will reach 14 metres by Thursday night - two metres above its major flood level.
Residents in western Sydney who endured floods in the Hawkesbury-Nepean region in March last year were told to leave on Wednesday evening after around 600 gigalitres of water flowed over the Warragamba Dam wall.
Ms Cooke said that would have a significant impact downstream with about 130,000 homes in the path of the overflowing dam.
The SES has also put out a blanket road weather Alert for all Sydney suburbs.
NSW Health said people isolating due to COVID-19 who need to evacuate their homes must leave.
It advises 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 the clean-up continues as the death toll rises, with four fatalities confirmed in the region's flood event so far.
The devastated town is also facing food shortages with supermarket shelves empty and petrol shortages. Residents are reporting ATMs have run out of cash.
Perrottet noted that Australian Defence Forces helicopters have been used to drop critical food supplies to evacuation centres.
"We've got a battle on two fronts. We're dealing with the immediate response of keeping people safe... through the operation centre and there is a substantial focus on recovery," he said.
Two women in their 80s and a man in his 70s were found dead in their flooded homes, while another man's body was found floating down a Lismore street.
A fifth man died on the Central Coast last Friday morning after his car was swept away in floodwaters.
