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(CNN) -- A huge tropical cyclone taking aim at Australia's Queensland state has strengthened to a Category 5 -- the highest designation on Australia's cyclone classification system -- and its landfall is "likely to be more life threatening than any experienced during recent generations," forecasters said.

Severe Tropical Cyclone Yasi has winds gusting up to 295 kph (183 mph) and is expected to maintain that intensity until making landfall in northeastern Queensland late Wednesday evening local time, the Australian Bureau of Meteorology said.

The cyclone threatens more devastation for Queensland, which already has been hit by deadly flooding in recent weeks.

"Yasi ... poses an extremely serious threat to life and property within the warning area, especially between Port Douglas and Townsville. This impact is likely to be more life threatening than any experienced during recent generations," the bureau says on its website.

Hundreds of patients have been evacuated from hospitals in the far north Queensland town of Cairns, and residents are being told to leave their homes as Cyclone Yasi closes in.

The storm is forecast to make landfall between Cairns and Innisfail.

Experts say it will bring torrential rain and cause a storm surge of up to 2 meters above the usual high tide line, flooding low-lying areas along the coast.

Queensland Deputy Police Commissioner Ian Stewart warned the state faces "one of the most significant weather events" in its history.

"This is a life-threatening storm, and people need to understand that they have a final window of opportunity to self-evacuate," Stewart said.

"In the hours that are left, people need to make their final preparations to either shelter in place, or evacuate."

The state's premier, Anna Bligh, has urged residents in the threatened areas to take sensible precautions and to stay inside once the storm hits.

"These will be highly destructive winds... they will be life-threatening if people make it their business to go outside in them.

"Loss of life and serious injury ultimately depends on people being sensible, listening to the warnings, taking advice and not treating this as a tourist event.

"It will be a display of the awesome power of nature, but it's not something you want to go outside and watch."

More than 250 patients -- including premature babies, those in intensive care, and on dialysis -- are being airlifted to facilities in Brisbane for treatment after the storm forced the closure of two hospitals in Cairns.

"We understand this will cause some anxiety, but it is only being done to make sure that very ill, very vulnerable patients are in the safest place possible," said Bligh.

More than 20 people died and thousands of homes were wrecked when severe flooding hit Queensland in January, affecting 3.1 million people.

Australian Prime Minister Julia Gillard has announced a one-off flood tax aimed at helping to pay for the estimated AUS $5.6 billion (US $5.58 billion) damage caused.

CNN's Jason Hanna contributed to this report.