"This is our worst fears," Deputy Premier Paul Toole said just before 6pm on Wednesday.
"It will get worse before it gets better."
He said any residents who were impacted by flooding in March last year, when about 460 gigalitres went over the Warragamba dam wall, should leave now, because the Georges River was likely to exceed the heights seen 12 months ago.
The Redbank dam is likely to fail, flooding homes near North Richmond, while rivers continue to rise.
"We do not want to see those situations where people are on the roofs of their houses waiting to be rescued," Mr Toole said.
The SES has about 31 evacuation orders and warnings in place, including the entire footprint of the Hawkesbury-Nepean flood of last year.
"This is going to be a tough night for residents in that area," Mr Toole said.
"Conditions are rapidly deteriorating," Bureau of Meterology meteorologist Dean Narramore says.
"A dangerous situation is evolving, we expect the rain to continue," he said.
SES Commissioner Carlene York said residents needed to start preparing to evacuate, with further evacuation orders likely to be issued overnight into Thursday.
"This a very serious situation and the rain has been a lot heavier than we had anticipated ... we have seen record levels that we haven't seen in recorded history," Ms York said.
"We don't want to leave it to the middle of the night ... you need to be prepared now."
High traffic on the SES website has people struggling to access information but Ms York said warnings would continue to be posted on social media and by other emergency services.
EVACUATION ORDERS AND WARNINGS FOR SYDNEY REGION:
* Parts of North Richmond in Sydney's northwest have been ordered to evacuate immediately, with the SES warning the nearby Redbank Dam is likely to fail.
* Parts of Chipping Norton, Lansvale, Milperra and Picton, southwest of Sydney, have been warned to prepare to evacuate also.
* Major flooding, similar to the 2021 floods, is expected along the Hawkesbury and Nepean rivers, at Menangle, North Richmond, Penrith and Windsor.
* Moderate flooding is also likely along the Georges River, at Liverpool and Milperra.
EVACUATION ORDERS AND WARNINGS FOR NSW NORTH COAST:
* Evacuation orders have been issued for parts of Casino, Maclean, Woodburn, Ballina, Ballina Island, Lawrence, Lennox Head, Ulmara, Coraki, Wardell, Tweed Heads, Bellingen, Murwillumbah, Lismore, Kyogle, Tumbulgum and more.
* Flood warnings have been issued for the Tweed, Wilson, Clarence, Bellinger and Hastings rivers.
