Greater Sydney is on high alert with the next 24 hours "absolutely critical" with torrential rain and flash-flooding forecast.
A fourth person has now been found dead in Lismore, as Greater Sydney braces for a "rain bomb" to unleash a deluge across the city.
NSW Police confirmed the latest death is a man in his 70s whose body was found in a flooded unit in south Lismore. His body has not yet been formally identified.
"This is terrible. This is terrible. One life lost is too many but these are communities that have been impacted, families, loved ones that have been impacted by these losses," NSW Deputy Premier Paul Toole told reporters on Wednesday.
These four deaths follow reports of an earlier death on Friday of a 54-year-old man in the state's Central Coast after his car got stuck in the floodwaters.
The Bureau of Meteorology is warning the intense rain forecast for Sydney and surrounds - as much as 200mm - will hit late on Wednesday and early Thursday.
Dean Narramore from the Bureau of Meteorology said Newcastle said the flash flooding is likely to be "life-threatening".
"Dangerous and treacherous driving conditions and damaging winds up to 90 kilometres an hour, particularly coastal locations could bring down trees and power lines," he said.
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."
Warragamba Dam has begun to spill, with WaterNSW warning it may continue for up to two weeks, threatening thousands of homes in the Hawkesbury and Nepean area.
The NSW State Emergency Service (SES) has asked people along the Hawkesbury and Georges rivers to review their evacuation kits and routes, with minor to major flooding expected.
Meanwhile, two elderly women were found inside their Lismore homes on Tuesday afternoon, both losing their lives while trapped in their flooded properties in the area in recent days.
A 54-year-old man who was feared missing on Sunday afternoon was found dead after being swept away by the Lismore floodwaters.
Mr Toole warned that more deaths in such a "catastrophic event" is inevitable.
"We know in realistic terms we may see more loss of life over the coming days," he said.
Lismore Mayor Steve Krieg echoed his sentiments, saying the Fire and Rescue crews search homes and businesses "probably with the sole purpose to make more grim discoveries".
"There's so many houses to go through. So many people still unaccounted for," he told the ABC on Wednesday.
"That is, unfortunately, the main job of the day today and we just really need people to stay away ... to let these people do the worst job imaginable."
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.
Ballina Hospital was evacuated on Tuesday night due to rising floodwaters, with 55 general medical and rehabilitation patients, moved to Xavier Catholic College.
An evacuation order was issued just after midnight for Ballina Island, with the SES warning the CBD expected to be inundated on Wednesday, isolating the community.
The BOM has issued several flood warnings for the Greater Sydney area, including the Upper Nepean at Menangle, the Hawkesbury River at Richmond and Windsor, and the Colo River at Upper Colo and the Georges River.
The communities in the path of the floods are the same ones devastated by floods just a year ago and thousands of people spent the night frantically sandbagging their homes and businesses.
People in low-lying areas around Windsor and Pitt Town, northwest of Sydney, have been warned to prepare to evacuate.
The slow-moving system arrived in Sydney late on Tuesday, with heavy rainfall over parts of the Hunter and Metropolitan, Illawarra, South Coast and parts of Central Tablelands and Southern Tablelands.
The intense rainfall and thunderstorms could also cause life-threatening flash-flooding, while wind gusts could exceed 90km/h in some areas.
Ms Cooke says it is too early to know exactly how many homes in western Sydney will be impacted by the overflowing Warragamba Dam.
"We know there's 130,000 residents in and around that area. It depends on exactly where the rain drops and how much it drops, so we are on high alert," she told the Seven Network.
While the rain has eased in the Northern Rivers region, the disaster is ongoing with some 35,000 people ordered to evacuate.
Lismore residents are no stranger to floods, but the most recent inundation of the town has been far worse than expected.
Flooding is seen in Chinderah, Northern NSW, Tuesday, March 1, 2022. More severe weather is expected along the NSW coast. (AAP Image/Jason O'Brien) NO ARCHIVING Source: AAP / JASON O’BRIEN/AAPIMAGE
Thousands of people have lost everything and are sheltering at evacuation centres.
Seventeen councils in northern NSW have been officially declared disaster zones.
The floods come against the backdrop of the landmark Intergovernmental Panel on Climate Change report released this week where scientists warned the nation is barrelling towards a future full of environmental disasters.
