Queensland Premier Anna Bligh said Severe Tropical Cyclone Yasi, a monster storm brewing on the Coral Sea, "may well be one of the largest and most significant cyclones that we've ever had to deal with".

It is on track to reach category four out of a five-point scale by the time it hits the Queensland coast early Thursday, and officials said it could eclipse Cyclone Larry, a 2006 storm that wrought some Aus$1 billion in damage.

"This is a very serious threat, I can't (overstate) the possible threat to people who live in this region," Bligh told reporters, warning of the risk of "significant flooding" in coastal areas.

Forecasters said La Nina-spurred Yasi was expected to pack winds in excess of 260 kilometres (162 miles) per hour and bring intense and prolonged rainfall across a wide swathe of the flood-hit northeast.

The Bureau of Meteorology issued a cyclone warning covering a mammoth section of Queensland's coast stretching some 900 kilometres (560 miles) -- nearly the length of Britain.

"Larry ... was certainly a very intense cyclone but it didn't have anywhere near the size or strong winds," bureau forecaster Ann Farrell said.

On Sunday a smaller category two storm, Cyclone Anthony, buffeted the region, tearing up roofs and downing trees and powerlines.

Authorities warned residents to stock up on food, water, batteries and other essentials in case they have to fend for themselves for a number of days after Yasi strikes.

Bligh approved forcible evacuation powers for police, urged people with low-lying waterfront homes to flee inland, and said nursing homes in the cyclone's path were being emptied.

Queensland is still reeling from a record deluge and floods that have destroyed tens of thousands of homes and killed more than 30 people this month, and Bligh warned of further pain for shattered communities.

"We couldn't rule out further flooding in areas that have already experienced significant flooding in the last four weeks if this cyclone behaves in the way it's currently predicted to do," she said.

"This is such a large system that the (weather) bureau does not expect it will dissipate quickly as it crosses, but for that rainfall to continue very significantly into those catchments that we've already seen very significant flooding in."

Cyclone Yasi's effects, including driving rain and winds, may be felt as far as 500 kilometres (310 miles) from the eye of the storm, forecasters said, a much bigger region than that hit by Larry.

If it reaches category three or four, it would knock out power to the region for up to four days and would pose a real danger to residents, said Whitsunday mayor Mike Brunker.

"We would start to worry more about structure damage, and yes, of course with that comes injury and loss of life," he told the state broadcaster ABC.

The Great Barrier Reef's Whitsunday resort islands began evacuating guests late Monday, and all ports from Cairns to Mackay were to close by Tuesday afternoon.

Shipments from the key coal-exporting region were just resuming after being shut down for weeks because of the flooding.

"At a time when we are just starting to get a bit of coal down the rail lines to the ports, clearly this is an unwelcome disruption to resuming normal service," said Michael Roche of the Queensland Resources Council.

The most destructive storm in Australian history was Cyclone Tracy, which in 1974 flattened Darwin, killing 65 people and leaving only about 400 of the northern city's 11,200 houses intact.

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