Evacuation warnings have been issued for residents northwest of Sydney amid heavy rainfall and flooding that left emergency services having to contend with snakes during one rescue on the NSW Mid-North Coast.
Major flooding is occurring along the Colo River, with residents on Monday night told to prepare to evacuate as waters continue to rise.
Major flooding similar to the 1988 and 1990 flood events is occurring along the Hawkesbury River while the Bureau of Meteorology is forecasting heavy rainfall to continue through Tuesday.
The State Emergency Service has told residents in several suburbs throughout Sydney's northwest to prepare to leave, with more than 18,000 having already been ordered to move in the city and northern NSW.
The SES has issued an evacuation order for low-lying properties in North Richmond and Agnes Banks due to rising flood water.
It has also issued a warning for moderate flooding along the Nepean River at Penrith with further rises possible.
Flood waters are also expected to affect the Upper Nepean River at Menangle Bridge.
The SES on Tuesday morning reported 9700 jobs across NSW including 870 flood rescues.
Onlookers observe the submerged New Windsor Bridge at Windsor, north west of Sydney, on Monday. Source: AAP
Fire and Rescue NSW crew were surprised by snakes jumping on to their life raft as they waded 1km in darkness to reach a family of two adults and four young children stranded at a house isolated by floodwater in Sancrox near Port Macquarie.
NSW's top forecaster says the record-breaking floods in many of the state's towns are the worst he's seen - and there's more torrential rain to come.
Homes have been swept away and thousands ordered to move amid relentless rain that has lashed the NSW coast for days.
Some locations have experienced almost a metre of rain in one week, BOM says.
"I've been a flood forecaster in the bureau for 20 years and this is probably the worst flooding that I've experienced and I've had to forecast," flood manager Justin Robinson said.
"We've got a flood watch that covers all the way from the Queensland border down to the Victorian border - all those coastal rivers.
"My thoughts really go out to those impacted communities and individuals."
More rain is on the way too, the BOM warns.
The state's south coast is due to cop a drenching and will get some of the heaviest falls on Tuesday.
The bureau is predicting widespread falls of between 100-200mm across the region, and 300mm in some parts.
The trough that has been causing the havoc is due to collide with another system coming in from the southwest.
That means Sydney and the Mid-North Coast could cop another 100mm in the next day or so, and a season's worth of rain is possible in the west.
Upstate communities are already facing the worst flooding conditions since 1929 and those along the Hawkesbury River are confronting the worst flooding since 1961.
A number of towns across the state have been isolated for days, some without fresh water or power.
Structures are seen during flooding in Richmond, north west of Sydney, on Monday. Source: Getty Images
Roads have been cut off, hundreds of homes inundated and more than 200 schools shut.
The renewed rainfall means the worst is potentially yet to come, Premier Gladys Berejiklian said.
"We're not through the worst of it potentially and that's why we need to brace ourselves," she told reporters.
"We have no illusions about how difficult the next few weeks and months will be."
Moderate flooding is occurring along the Macleay River at Kempsey and Smithtown where it has peaked, but the bureau predicts further rises on Tuesday.
Major flooding is occurring at Wollombi in the Hunter Valley, while moderate flooding is still plaguing Taree and Gloucester.
Flooded Old Hawkesbury Road near Pitt Town and Windsor in the North West of Sydney, on Monday. Source: AAP
Inland, the Macquarie River levels have peaked at Bathurst, with minor flooding continuing.
There has been some good news, with those evacuated from the western part of Jamisontown and Penrith, and the northern end of Mulgoa are among those given the all-clear to return home.
The Australian Defence Force will provide two search and rescue helicopters operating out of the NSW south coast for 24 hour operations.
