Riverbanks have burst, dams are spilling over, roads are cut and residents are on alert for evacuation as NSW faces its worst rain event in almost three decades.
Premier Gladys Berejiklian has told residents on the mid-north coast, the Hunter, the Central Coast, metropolitan Sydney and the Warragamba Dam catchment area to be ready to leave their homes.
"This will be a deep-seated, extreme weather event ... It's not going to be an easy week for us," she told reporters on Saturday.
"None of us are out of the woods while the storm front is moving south. The rain may not stop until Thursday or Friday. I hope those predictions are wrong."
Ms Berejiklian said weather modelling included an up to one-in-25-year event.
People must be ready to leave their homes immediately, she said.
The State Emergency Service has advised Picton residents in the Warragamba Dam catchment to get ready to leave when instructed to do so.
For NSW residents not in immediate danger, the premier urged them to restrict their movements and heed all warnings.
Nine evacuation are centres open in NSW - seven on the mid-north coast and two in the Hunter region.
Emergency Services Minister David Elliott said the State Emergency Service's 500 flood rescues was "completely unacceptable".
"That's 500 crews that have put their lives at risk because people have not heeded the warnings," he said.
Warragamba Dam, the Sydney's main reservoir, began overflowing just after 3pm on Saturday and the spill is expected to increase into Sunday, Water NSW says.
Other dams such as the Nepean, Cataract, Cordeaux and Avon are also expected to reach capacity and begin spilling on Saturday.
The lower Blue Mountains are forecast to see the worst of the rain on Saturday and into Sunday.
Search teams are on the water off the coast of Coffs Harbour looking for a bodyboarder in his 60s who went missing mid-afternoon.
And in Katoomba, in the upper Blue Mountains, two bushwalkers were cut off by floodwaters on the Six Foot Track with NSW Ambulance saying they were "extremely fortunate" to have survived.
The rescue required four crews and conditions were difficult with some roads impassable because of flooding.
There are multiple flood warnings for rivers including the Nepean, Hawkesbury and Colo.
Major flooding is possible at North Richmond, Windsor and Sackville, from late Saturday, the SES said.
There are concerns for communities along the Georges River, an urban river in the city's south, and on the Hawkesbury/Nepean river system to the north and west of Sydney.
"It's a very dynamic and evolving flood situation and we could see some very deep and rapidly responding rivers with very high levels," BOM national flood services manager Justin Robinson told reporters.
River systems on the mid-north coast that have already flooded communities are expected to cop more rain on Saturday night and on Sunday, as Sydney's river systems swell dramatically.
Senior BOM climatologist Agata Imielska said greater Sydney could expect about 100mm of rain during the next 24 hours or so, but the lower Blue Mountains is expected to get 200-300mm.
The band is moving down from the Kimberley and will reach eastern NSW from Monday night, when it will "link up" with the slow-moving coastal trough that is causing the flooding in NSW.
Parts of Port Macquarie, and several nearby towns, have flooded and further south, Taree is in the grip of a flood rivalling its worst on record 92 years ago.
The federal government has announced financial disaster recovery assistance for Armidale, Bellingen, Central Coast, Cessnock, Clarence Valley, Coffs Harbour, Dungog, Kempsey, Lake Macquarie, Maitland, Mid-Coast, Nambucca Valley, Newcastle, Port Macquarie-Hastings, Port Stephens and Tenterfield local government areas.
"A range of practical assistance measures are now available to help people get back on their feet and help councils get on with the clean-up and repairs to infrastructure," Emergency Management Minister David Littleproud said.
Prime Minister Scott Morrison said the scenes across NSW were "absolutely heart-breaking" and government was ready to assist, including summoning the Australian Defence Force.
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 on Saturday evening.
Vaccines were supposed to have been delivered by the end of the weekend for the next phase of the immunisation program.
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 SES on 132 500.