Stranded residents sheltered on a hospital roof as flash floods swamped northeastern Australia on Monday, with raging waters severing roads and flushing crocodiles into towns. 

Rescue teams evacuated more than 300 people overnight, police said, and military helicopters were dispatched to help inundated areas cut off by the floods.

Damage was reported along an expanse of coastline that stretched about 400 kilometres (250 miles) across northern Queensland.

With another deluge expected Monday, Queensland Treasurer Cameron Dick said the unfolding disaster would have a "billion-dollar impact" on the state. 

Nine people, including a seven-year-old patient, huddled for safety overnight on the roof of a hospital in the largely Aboriginal settlement of Wujal Wujal.

"We know that those people are in a desperate way now," said Kiley Hanslow, the chief executive of the Wujal Wujal Aboriginal Shire Council.

Police said the group eventually clambered to a safer location before flood waters rose again on Monday afternoon.

Surrounded by a mountainous hinterland of tropical rainforest, hard-to-reach Wujal Wujal is one of the most disadvantaged regions in Australia. 

Hanslow told national broadcaster ABC the town centre was a "sea of dirty water and mud."

"There's also crocodiles swimming around in that water now," she added.

Queensland Police Commissioner Katarina Carroll said flood waters would likely wash "crocs and all sorts of other things" into residential areas. 

"You would recall from past events we've had sharks, crocs, you name it," she told reporters. 

Wildlife officers in the rural town of Ingham used a lasso to catch a crocodile that had been bathing in shallow water next to houses. 

- 'Next level' rains -

The state of Queensland has been pummelled by damaging winds and driving rain in the wake of Tropical Cyclone Jasper, which barrelled in off the Coral Sea late last week.

The tourist hub of Cairns has been almost completely surrounded by the floods, which have washed over the major highways running into the city of 150,000 people.

On the outskirts of Cairns, locals used ropes to try and rescue helpless livestock swept away by the torrents. 

Stagnant pools of flood water lapped against the wings of planes parked at the Cairns international airport.  

"This level of rainfall is next level," Queensland Premier Steven Miles told reporters on Monday.  

"We deployed literally every boat we could get our hands on in Cairns to evacuate those who couldn't safely evacuate themselves."

Miles said authorities were starting to worry about the dwindling supplies of clean drinking water, urging people to conserve "as much as they can". 

The Australian military used two heavy lifting Chinook helicopters to ferry supplies and personnel into flood-stricken towns.  

Police Commissioner Carroll said authorities were thankful they had not seen any deaths or serious injuries during the emergency so far. 

Researchers have repeatedly warned that climate change amplifies the risk of natural disasters such as bushfires, floods and cyclones.