SYDNEY, MARCH 03

Authorities in Australia issued more orders for people to leave their homes on Thursday after heavy rain triggered flash floods in its largest city, with officials warning of worse to come and some 500,000 people likely to face orders to evacuate.

Australia's east coast has been battered by a severe weather system that has cut off entire towns and submerged hundreds of homes and farms as it has moved south from Queensland state over the past week.

Thirteen people have been killed since the deluge began a week ago.

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

Perrottet said the floods would likely be worse in some places than floods last year, which were the worst in 60 years.

The second year of flooding comes as the La Nina weather pattern, typically associated with increased rainfall, has dominated Australia's east coast over the summer. Rivers and catchments were already near capacity before the latest drenching after steady rains over the last few weeks.

Authorities said not as much water was expected to overflow from the Warragamba Dam, Sydney's major water supply, as earlier feared as rains slightly eased.

Some Sydney suburbs got more than 100 mm (4 inches) of rain over the past 24 hours and the weather bureau said more was on the way with some places to get up to 150 mm on Thursday.

March's mean rainfall in Sydney, home to more than 5 million people, is around 140 mm.

Satellite images showed the storm drifting away from Sydney but several suburbs in the city's west are still battling rising waters. Tens of thousands of residents were ordered to evacuate in the middle of the night on Wednesday. Read full story

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

'We are shattered and my heart is aching. Not sure when roads will be clear ... what a week!,' the owner of a cloth store wrote in a Facebook post and posted pictures of a pile of debris in her flood-hit store.

Heavy rainfall and wild winds, meanwhile, returned to southeast Queensland, already devastated by record flooding in recent days, hampering relief efforts with the weather bureau predicting 'incredibly intense rain' there on Thursday.

'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.'