Major flooding is expected along the Hawkesbury and Nepean and Georges rivers, at Menangle, North Richmond, Penrith and Windsor on Thursday.
The State Emergency Service received 3166 calls for help in 24 hours and overnight issued more than 20 evacuation orders to tens of thousands of people living in Sydney's west, northwest and southwestern suburbs including Windsor, North Richmond, Camden and Milperra.
The Bureau of Meteorology is warning of life-threatening flash flooding and damaging winds with peak gusts in excess of 90 km/h.
SES assistant Commissioner Sean Keans says an east coast low off Newcastle will bring heavy rain to the Hunter, the Central Coast and Sydney, the Illawarra and the South Coast on Thursday morning.
"That's going to bring with it up to 250mm of rain over a two to three hour period which can be really dangerous and it's also going to bring strong wind, so we are asking people if they can avoid unnecessary travel," he told Sydney radio 2GB.
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.
"The flooding is going to be higher this time," Mr Keans warned.
More than 250 schools across the state are closed and the SES is urging people to avoid driving as flooded roads and reduced visibility make conditions dangerous.
Deputy Premier Paul Toole said on Wednesday night the Georges River was likely to exceed the heights of the 2021 flood and residents were in for a "tough night".
"If you were effected by flood in 2021 get out now," he said.
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.
About 130,000 homes are in the path of the overflowing dam, and Mr Toole wanted them to be empty before floodwaters hit.
"Get out now ... We do not want to see those situations where people are on the roofs of their houses waiting to be rescued," he said.
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 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.
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.
