Fresh evacuation orders were issued for thousands of Sydney residents on Monday after relentless rains flooded several suburbs in Australia's largest city, with officials warning of more wild weather over the next 12 hours.

An intense low-pressure system off Australia's east coast is forecast to bring more heavy rain through Monday across the southern regions of New South Wales even as several places in the state were hit with about a month's rain over the weekend.

About 100 millimeters (4 inches) of rain could fall in the next 24 hours over a wide swathe of regions in New South Wales, from Newcastle to the south of Sydney, a distance of more than 300 kms (186 miles), the Bureau of Meteorology said.

"If you were safe in 2021, do not assume you will be safe tonight. This is a rapidly evolving situation and we could very well see areas impacted that have never experienced flooding before," New South Wales emergency services minister Steph Cooke said in a televised media briefing on Sunday evening.