Conditions were rapidly deteriorating by mid-afternoon on Wednesday as the effects of an east-coast low, which caused deadly floods in Queensland and northern NSW, hit the state's capital.
The Bureau of Meteorology's Dean Narramore said catchments already drenched by weeks of consistent rain were being bombarded with 50 to 100mm of fresh downpours, and it was expected to continue into the morning.
"A dangerous situation is evolving," he warned.
The State Emergency Service (SES) later issued major flood warnings for the Hawkesbury, Nepean and Georges rivers, with peaks expected early this morning.
With the SES website crashing due to "unprecedented traffic", it was up to the Hawkesbury SES Facebook page to deliver the first evacuation order of the day at 5pm.
Those in low-lying parts of North Richmond, south of the Redbank Dam, were told to "get out now".
"This is to ensure that you're not putting your life at risk or even putting our SES, or emergency services', lives at risk," Deputy Premier Paul Toole said.
The North Richmond Bridge was closed, and people living in the Hawkesbury and Nepean floodplains were warned conditions could match the traumatic 2021 floods.
Residents along the Nepean and Hawkesbury rivers were told to prepare to evacuate.
SES Commissioner Charlene York said that had put hundreds of thousands of people on notice of the "very serious situation" at hand.
York said there were already signs of flooding in the state's south, from the Illawarra to the South Coast, including a landslide at Kiama.
By 8pm, six new evacuation orders were given for densely populated suburbs along the Georges River near Bankstown and Liverpool.
Residents were told to leave by 3am the next morning, but people in parts of Camden, further south-west of the city, were told to evacuate immediately just before 8:30pm.
Then at 10:20pm, an evacuation order was given for several towns in the Hawkesbury region between Leets Vale and Agnes Banks.
In the town of Ballina, in the state's north, floodwaters were receding but many communities remain isolated and effectively homeless.
Amanda Causley is overseeing more than 20 evacuation centres across Tweed Heads, Lismore, Ballina and Grafton and said they have had more than 4,000 people staying in them.
She said resupplying evacuation centres with food and water was a priority.
"There are a lot of isolated areas … the issue at the moment is getting into towns, and we are working through that, and we do have air support to do food drops," she said.
"The evacuation centres will stay open at least for another few days, we need to wait for those waters to recede and then reassess."
Rudy Maxwell is one of hundreds still isolated at an evacuation centre at Coraki, south of Lismore.
She said people, including the elderly, have been bunking down on the floor and supplies of food and medications have been an issue.
"It's very difficult and uncomfortable for them … we've had food helicoptered in and also fuel," she said.
"The priority now is, there are a few people who are unwell, and the nurses and the emergency staff are hoping to get them out.
"There's a family bunkered down in the pub at Coraki and they are looking after a four-month-old baby there."
Fatalities in the northern NSW disaster zone continue to mount, with four people confirmed to have died as of Wednesday night.