Residents of Sydney and the NSW mid-north coast have been given middle-of-the-night evacuation orders as rivers in the state threatened to burst their banks.
Flooding in the areas near the Hawkesbury Nepean Valley was the worst since 1961.
The Nepean River at Penrith peaked at 10.05 metres on Sunday night, its highest level in 40 years, the Bureau of Meteorology (BOM) said in a statement early Monday morning.
Kate Dukes, 45, who lives just a block from the Nepean River in Penrith, said she and her family decided not to evacuate despite warnings to do so, because their home is situated at a high enough point to avoid the floodwater.
"Our elderly neighbours were evacuated, and it was good to see them leave, because if we were flooded in and unable to leave the street that would have been a major concern," she said.
Watching the river approach its peak on Sunday night felt monumental, she said.
"It felt historical to watch, these things don't happen very often."
The Hawkesbury River at Windsor is expected to peak around 13 metres by Monday evening, with major flooding expected, the bureau warned.
Warragamba Dam, which began spilling over on Saturday afternoon, is expected to continue overflowing on Monday.
The BOM warned the danger isn't confined to the coast – the state's west could see increased rain from Monday as well.
People in the Hawkesbury and surrounding areas were told to evacuate by 9am on Monday, including the Windsor CBD, McGraths Hill, South East Windsor, Wilberforce, Marsden Park, Riverstone and Schofields.
The Castle Hill RSL at 77 Castle Street was set up as an evacuation centre.
On the mid-north coast, residents of Kempsey were awoken just before midnight and told to evacuate immediately.
Evacuations were already underway in parts of Western Sydney over the weekend, as streets became submerged in rainwater and low-lying homes drenched.
There have been 19 evacuation orders listed on the State Emergency Service website in the past three days, along with evacuation warnings for several other areas across the state.
Sunday saw rolling evacuation orders for Western Sydney.
Shortly before 2am, a number of suburbs were told to evacuate immediately, including Pitt Town North, Pitt Town Bottoms, Cornwallis and Grono's Point.
At dawn, people who live in low lying areas of North Richmond were told to leave their homes. Freemans Reach residents were told to get out by 9am.
Residents of low lying areas in Agnes Banks were told to evacuate by midday, and people in western Penrith by 4.30pm.
People who live in Jamisontown, Regentville and low lying parts of Mulgoa Creek were told to evacuate by 9pm on Sunday.
Those orders followed previous evacuation orders over the weekend in suburbs including low lying areas of Taree CBD, Taree Estate, Dumaresq Island, Cundletown, Central Wingham and Wingham Peninsula.
Prime Minister Scott Morrison said the floods across NSW was "another testing time for our country".
"There are very, very serious and very severe storms and floods," Morrison told 2GB.
"It is a matter of pulling through, together, once again."
Morrison said the NSW government was yet to request support from the defence force, although helicopters were on standby to assist with search and rescue operations.
"We expect later today we will start getting requests for the recovery operations on the other side so we're readying ourselves for that.
He said emergency payments of A$1000 for adults and A$400 for children were already in place for people in affected areas via Services Australia.