Earlier in the day, she urged people to reconsider holiday travel, with the rough weather having hit at the beginning of school holidays.

"This is a life-threatening emergency situation," Cooke said.

More than 200 millimeters (8 inches) of rain have fallen over many areas with some hit by as much as 350mm, the Bureau of Meteorology said, warning of flood risks along the Nepean and Hawkesbury Rivers.

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Camden in southwest Sydney was underwater, and the weather bureau predicted water levels in the areas of North Richmond and Windsor northwest of Sydney would peak at higher levels than in the past three major flood events since March 2021.