More than 30,000 residents of Sydney and its surrounds were told to evacuate or prepare to abandon their homes Monday as Australia's largest city faces its fourth, and possibly worst, round of flooding in less than a year and a half.

Days of torrential rain caused dams to overflow and waterways to break their banks, bringing a new flood emergency to parts of the city of 5 million people.

"The latest information we have is that there's a very good chance that the flooding will be worse than any of the other three floods that those areas had in the last 18 months," Emergency Management Minister Murray Watt said.

The current flooding might affect areas that were spared during the previous floods in March last year, March this year and April, Watt added.

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

"You'd probably expect to see that number increase over the course of the week," Perrottet said.

Emergency services made numerous flood rescues Sunday and early Monday and were getting hundreds more calls for help.

Australia's Bureau of Meteorology manager Jane Golding said some areas between Newcastle, north of Sydney, and Wollongong, south of Sydney had received more than a meter (39 inches) of rain in the previous 24 hours. Some has received more than 1.5 meters (59 inches).

Those totals are near the average annual rainfall for coastal areas of New South Wales.

"The system that has been generating this weather does show signs that it will ease tomorrow, but throughout today, expect more rain," Golding said.

Rain was forecast across New South Wales's coast, including Sydney, all week, she said.

The Bureau of Meteorology says up to 12 centimeters (4.7 inches) of rain could fall in Sydney on Monday.

The flooding danger was highest along the Hawkesbury River, in northwest Sydney, and the Nepean River in Sydney's west.

The bureau Monday afternoon reported major flooding at the Nepean communities of Menangle and Wallacia on Sydney's southwest fringe.

Major flooding also occurred on the Hawkesbury at North Richmond on Sydney's northwest edge. The Hawkesbury communities of Windsor and Lower Portland were expected to be flooded Monday afternoon and Wisemans Ferry on Tuesday, a bureau statement said.

State Emergency Services Commissioner Carlene York said strong winds had toppled trees, damaging rooves and blocking roads. She advised against unnecessary travel.

Off the New South Wales coast, a cargo ship with 21 crew members lost power after leaving port in Wollongong on Monday morning. It was anchored near the coast and tugboats were preparing to tug it into safer, open waters.

The ship has engineers on board capable of repairing the engine, port official John Finch told reporters. "Unfortunately, we just happen to be in some atrocious conditions at the moment," he said, describing 8-meter (26-foot) swells and winds blowing at 30 knots (34 mph).

An earlier plan to airlift the ship's crew to safety was abandoned because of bad weather.

Repeated flooding was taking a toll on members of a riverside community southwest of Sydney, said Mayor Theresa Fedeli of the Camden municipality where homes and businesses were inundated by the Nepean River over Sunday night.

"It's just devastating. They just keep on saying 'devastating, not again,'" Fedeli said.

"I just keep on saying ... 'We've got to be strong, we will get through this.' But you know deep down it's really hitting home hard to a lot of people," she added.

Perrottet said government and communities needed to adapt to major flooding becoming more common across Australia's most populous state.

"To see what we're seeing right across Sydney, there's no doubt these events are becoming more common. And governments need to adjust and make sure that we respond to the changing environment that we find ourselves in," Perrottet said.