Incessant rain is expected to drench parts of the state for days, with Greater Sydney's Hawkesbury Nepean Valley - incorporating major urban centre Penrith - also set to cop its worst flooding in 50 years.
NSW's mid-north coast is experiencing a "one-in-100-year" weather event while parts of western Sydney and the Hawkesbury are dealing with the worst floods the area has seen in 50 years as extreme downpours continue to batter parts of the state.
Dozens of flood warnings, evacuation warnings and evacuation orders are in place across NSW while the state and federal government have declared natural disaster zones in flood-hit areas, allowing the release of critical disaster relief funding.
Up to 1,500 SES volunteers across the state have responded to 7,000 calls for assistance and made 650 flood rescues since Thursday.
While heavy rain was expected to continue over parts of the state until Wednesday, NSW Premier Gladys Berejiklian said the disaster was not expected to worsen on the mid-north and central, with the main areas of concern now being the communities around Richmond, Hawkesbury Valley and parts of Western Sydney.
Warragamba Dam, Sydney's main water source, spilled over on Saturday afternoon, causing river levels to rise along the Nepean and Hawkesbury. Suburbs in the region could now experience flood levels not seen since the 1961 flood, with thousands more people expected to be evacuated over coming hours.
"Yesterday we were hoping it will only be a one-in-20-year event, now it looks like a one-in-50-year event," Ms Berejiklian told reporters on Sunday.
"Potentially another 4,000 people may be asked to evacuate in and around the Hawkesbury region, parts of western Sydney. We will know in the next few hours whether that is the case."
Thirteen evacuation centres have been set up across the mid-north coast, the Hunter, Richmond and the Hawkesbury, Ms Berejiklian said, advising those evacuated to stay with family or friends as a first option.
The NSW premier also thanked South Australia, Queensland and Victoria for offering assistance.
The Bureau of Meteorology national flood services manager Justin Robinson said the areas around the Nepean River could expect flood levels not seen since the 1960s.
"We are expecting river levels at Penrith to be levels near the 1961 flood. To give you some context, that is bigger than the February 2020 flood. It is bigger than the 1988 flood. It is bigger than the 1990 flood, and it is bigger than the 1964 flood. It is one of the biggest floods we are likely to see for a very long time."
Mr Robinson said floodwaters at Penrith were expected to continue to rise throughout Sunday, with floodwater levels expected to move downstream and impact communities at North Richmond, Windsor, Sackville among others.
He urged residents in the area to keep track of updates on the BOM’s website.
SES assistant commissioner Dean Storey described the unfolding disaster as a "serious, potentially life-threatening weather event" that will "continue to affect a large part of the state for the coming days".
"The key message of the State Emergency Service at this time is that all communities need to be aware of their risk. Have a plan and prepare accordingly."
Mr Storey advised people in hard-hit areas to prepare themselves, their families and their pets for potential evacuation, or if their area was already subject to an evacuation order, to "leave immediately" and follow the direction of authorities on the ground.
"This is a dynamic weather situation," he said.
The Bureau of Meteorology said rainfall was expected to "really intensify" on the north coast over coming days.
"The North Coast today that is getting hit once again. Tomorrow we are expecting it to really intensify for that particular region, so be very mindful for the communities out there," a BOM spokesperson told the media on Sunday.
"You have already experienced very dangerous conditions, and they are going to be treacherous yet again, so keep a close eye on warnings, and again, expect those river rises and flood risk to continue."
The natural disaster zone extends from the Coffs Harbour and Grafton area in northern NSW into the west to Cessnock and Dungog and up to the Central Coast.
The Bureau of Meteorology has issued flood warnings for more than a dozen rivers across the state.
Residents in Pitt Town Bottoms, Pitt Town North, Cornwallis, North Richmond, Grono's Point, Freemans Reach and Agnes Banks west of Sydney were told to evacuate in the early hours as the Hawkesbury River flooded.
Earlier on Sunday, SES Deputy Commissioner Daniel Austin said the service will be working beyond Easter on the post-flood clean up effort and restoration of key services.
"Whilst a couple of areas have seen some receding floodwaters overnight and into this morning, that heavy rainfall returning to those locations today will likely lead to additional peaks on those river systems," Mr Austin told ABC TV.
"We're planning well beyond Easter for our own operations ... just because the rain may stop on Thursday, the rivers naturally don't go back to their normal state and then there's going to be an extended recovery period."
Moderate flooding also continues along the Colo River in the Blue Mountains, with farmers near the river told to be on alert for flooding and be ready to move livestock.
Bellingen residents and people at a tourist park were also advised to evacuate because of the risk of flooding along the Bellinger River.
There were also flood warnings in place for Hunter River, the Myall River and the Camden Haven River on the NSW mid-north coast and the Orara River in the state's Northern Rivers district.
Parts of Port Macquarie, Taree and nearby towns have also flooded.
Meanwhile, a bodyboarder in his 60s went missing off the Coffs Harbour coast on Saturday afternoon and crews resumed the search on Sunday.
Two stranded bushwalkers were also rescued in the Blue Mountains on Saturday as they were returning from a camping trip at Katoomba.
In Taree, there were reports of a river whisking a cow out to sea before it washed up on the beach. She had reportedly been rescued and returned to her paddock.Strong winds have also caused damage, with a small tornado ripping through Chester Hill High School in Sydney's west on Saturday.
The rain and foul weather is being caused by a coastal low-pressure trough combined with a strong high pressure system in the south.
Prime Minister Scott Morrison said the scenes across NSW were "absolutely heartbreaking" and the government was ready to assist.
The federal government has announced financial assistance for more than a dozen local government areas.
The COVID-19 vaccine rollout in Sydney and many parts of NSW has been delayed because of the extreme weather, the Department of Health said.
The Public Information and Inquiry Centre provides information about the severe weather at any time of day on 1800 227 228. For emergency help in floodwaters, call the NSW SES on 132 500.