30,000 residents asked to evacuate as Sydney faces fourth flooding emergency in 18 months

More than 30,000 residents of Sydney and its surroundings have been told to evacuate or prepare to abandon their homes on Monday as Australia's largest city braces for what could be its worst flooding in 18 months.

Parts of the city of 5 million people are facing a fourth flooding emergency in a year and a half after torrential rain since Friday caused dams to overflow and waterways to break their banks.

New South Wales state Premier Dominic Perrottet said 32,000 people were impacted by evacuation orders and warnings.

The towns of Camden and Windsor along the Hawkesbury river were particularly impacted overnight.

The Bureau of Meteorology says there could be up to 120 millimetres of rain in Sydney on Monday.

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Emergency services had made 116 flood rescues in recent days, 83 of them since 9 p.m. Sunday, the Premier said. Hundreds more requests for help were made by Monday morning.