Severe Tropical Cyclone Yasi strengthened to a category five storm from category four as it menaced the country's populous east coast, where it was set to hit late Wednesday or early Thursday, the Bureau of Meteorology said.

"Severe Tropical Cyclone Yasi is a large and very powerful tropical cyclone and poses an extremely serious threat to life and property within the warning area, especially between Port Douglas and Townsville," the bureau said.

"This impact is likely to be more life-threatening than any experienced during recent generations," it said in a cyclone warning that upgraded the strength of the looming storm.

The monster storm was over the Coral Sea about 650 kilometres (400 miles) off the coast early Wednesday, moving in a west-southwesterly direction as thousands of locals and tourists fled the coast of northern Queensland state.

Yasi is expected to generate destructive winds with speeds of up to 280 kilometres (175 miles) per hour, expected to hit large swathes of the coast which is dotted with towns and tourist resorts, meteorologists warned.

Coastal residents, particularly between the cities of Port Douglas and Townsville, were also warned of an "extremely dangerous" storm tide that was likely to cause extensive flooding as the cyclone crossed the coast.

Thousands of residents and holidaymakers have flown out of the region, while two hospitals in Cairns were evacuated and shuttered, with patients airlifted by the air force to the city of Brisbane.

But airports in Cairns and other cities in the area were set to close Wednesday morning as winds picked up strength, while frightened residents who are staying battened down and prayed they would be safe.

"I'm trying to convince the family to head up into the mountains," Cairns resident Simon Fuller told public broadcaster ABC.

"It's awful in Cairns right now, the air is pushing down, it's like being under a wet doona (duvet)."

The storm's size and power dwarfs Cyclone Tracy, which hit the northern Australian city of Darwin in 1974, killing 71 people and flattening more than 90 percent of the city's houses.

It will also be far stronger than the mid-category four Cyclone Larry that caused Aus $1.5 billion ($1.5 billion) of damage after hitting agricultural areas around the town of Innisfail, just south of Cairns, in 2006.

Forecasters said it could take at least 24 hours for Cyclone Yasi to weaken after it makes landfall.

"If you're bunkering down in the regions it's going to be quite frightening and it's going to go on and on for quite some time," meteorologist Gordon Banks told the ABC.

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