Roads have been cut off, with 18 evacuation orders in western Sydney alone.
"This is a life-threatening emergency situation," Stephanie Cooke, emergency services minister for the state of New South Wales, said.
The area was hit by flash flooding in March, killing 20 people. "We are now facing dangers on multiple fronts - flash flooding, riverine flooding and coastal erosion," Ms Cooke in a media briefing.
Ms Cooke said this was a "rapidly evolving situation" and warned that people should be "prepared to evacuate at short notice". Authorities are pleading with people to heed advice to leave their homes
There had been 83 flood rescues by emergency services in the past 24 hours alone, she said.
Experts say the flooding emergency has been worsened by climate change and a La Niña weather phenomenon. A La Niña develops when strong winds blow the warm surface waters of the Pacific away from South America and towards Indonesia. In their place, colder waters come up to the surface.
In Australia, a La Niña increases the likelihood of rain, cyclones and cooler daytime temperatures.
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