The flames appeared to be dying down as firefighters poured in water from multiple hoses two hours after the fire took hold in a building near the Central Station railway hub.

Zoo keepers tranquilised and returned one cub while the remaining four made their way back of their own accord.

Even as Australian business groups and unions assemble at the national capital for the summit, Sydney, is facing a protracted strike by train employees and protests by nurses seeking higher wages.

Water levels in Hawkesbury River in Sydney's west began to recede bringing relief to residents in Windsor, one of the worst-hit suburbs.

About 50,000 residents in New South Wales, most in Sydney's western suburbs, have been told to either evacuate or warned they might receive evacuation orders, authorities say.

An intense low-pressure system off Australia's east coast is forecast to bring heavy rain across New South Wales after several places in the state were hit with about a month's worth over the weekend.

About 100mm of rain could fall in the next 24 hours over a wide swathe of regions in New South Wales, from Newcastle to the south of Sydney.

Climate change is a contentious issue in Australia which is one of the world's biggest carbon emitters on a per capita basis and is the world's top exporter of coal and liquefied natural gas.

Heavy rains lashed Sydney with some suburbs having received up to 200mm, exceeding March's mean rainfall of around 140mm and triggering snap evacuation orders in the southwest of the city.

The death toll from the deluge rose to 18 after a man was found dead in a car swept away in floods in Queensland on Sunday.

Authorities issued severe rain and wind warnings for a 400km stretch of the coast as water levels rose rapidly – including in suburbs around Sydney, Australia's largest city and home to five million people.

The country'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