Jane Golding, from the Bureau of Meteorology, said some areas of Greater Sydney had received more rain than they would for the entire month of July.

"The numbers are comparable to (the rainfall in) March. What is different in this event is that the rain was stacked up over several days, and that increases risk of how rivers respond," she said.

Along with heavy rain, winds of up to 100 kilometers (62 miles) an hour have been recorded on land, and gale force warnings are in place off the coast, where there are waves of up to five meters (16 feet).

The dangerous conditions forced authorities to abandon efforts to rescue 21 crew members trapped on a Hong Kong-registered cargo ship, the Portland Bay, which was stranded without power off the New South Wales coast. Instead state police said a tug boat was dispatched to tow the ship further out to sea, where the Australian Maritime Safety Authority would attempt to restore its power.