Authorities in Australia issued more orders for people to leave their homes after heavy rain caused flooding in its largest city, Reuters reported.

Australia's east coast has been hit by severe weather that has cut off entire towns and flooded hundreds of homes and farms.

Thirteen people have died since the flood began a week ago.

"We do believe that things will get worse," New South Wales state Premier Dominic Perrottet told reporters, adding that some half a million people would be affected by evacuation orders and warnings.

In some places, flooding is likely to be worse than last year's floods, which were the worst in 60 years.

Some Sydney suburbs got more than 100 mm (4 inches) of rain over the past 24 hours.

The average rainfall in March in Sydney, home to more than 5 million people, is around 140mm.

Satellite images showed the storm was moving away from Sydney, but several suburbs in the west of the city were still struggling with rising waters. Tens of thousands of residents were ordered to evacuate in the middle of the night on Wednesday.

In Lismore, the north New South Wales town worst hit by the floods, business owners began taking stock of the damage as waters receded.

Meanwhile, heavy rains and wild winds have returned to south-east Queensland, already devastated by record flooding in recent days, hampering relief efforts.

"Conditions are going to be unstable for the next 24 to 48 hours," Queensland Premier Annastacia Palaszczuk told reporters. "I haven't seen storms and floods like this, all being thrown at us at once."