Cyclone Yasi is expected to be larger and more intense than 2006's devastating Cyclone Larry when it slams into the north Queensland coast sometime tomorrow night.

Queensland Premier Anna Bligh delivered the stark assessment as she warned residents that today was their last chance to abandon homes in low-lying suburbs in the path of the possible category four system.

Ms Bligh warned the cyclone could be the worst the state's ever seen, with the potential to cause "powerful and deadly" flash flooding, especially in low-lying areas near the coast.

Additional commercial aircraft were flying into the area today, to take residents who wished to flee the region.

"Today may be the last chance to make the decision to leave", warned Ms Bligh.

The cyclone is expected to pack winds of up to 280 kilometres per hour when it hits the coast somewhere between Cairns and Innisfail at about 1am (local time) on Thursday.

"This is not only now tracking as more intense than Cyclone Larry, it is significantly larger than Cyclone Larry," Ms Bligh told a press conference today.

She said the storm's eye could take more than an hour to pass and warned that the strongest wnds would hit after the eye had passed.

"This eye could last for more than an hour and at the end of that period the next thing that will be felt is the strongest possible winds," she said.

Evacuations underway

Evacuations are underway as the storm continues its charge towards the north Queensland coast.

State disaster co-ordinator Ian Stewart said a large number of people were self-evacuating.

"A large number of people are determined to self-evacuate out of the danger areas by driving out," Mr Stewart told reporters.

"We ask people don't panic buy in terms of petrol.

"Those people (who are driving out of the area) ... we would like you to be very patient and very careful."

He said a large number of traffic police were assisting traffic driving south or west out of the region.

But Mr Stewart said the last thing needed was precious resources wasted on having to attend to traffic snarls or squabbles.

Queensland chief medical officer Jeannette Young told reporters authorities were erring on the side of caution by evacuating patients from the hospitals.

"We know we can do it safely with the assistance of ADF and we know we have the capacity in Brisbane to take those patients," she said.

"I'm confident it's the right thing to do by those patients."

She said the evacuated patients ranged from people in intensive care, mothers preparing to give birth, premature babies and patients receiving dialysis.

"We do this every single day in Queensland, we have a very big state and we move patients around the state all the time," she said.

"We'll be putting our doctors and nurses on those planes to move those patients safely.

Mr Stewart said he would on Tuesday afternoon meet with mayors about whether forced avacuations would be needed.

He encouraged people to get as far south as Mackay if they could, with that city expected to experience conditions like a category one storm.

"In reality, we would like people to get as far south as possible, as quickly as possible, without of course breaking the rules," he told reporters.

"Mackay is probably a target area for ... complete safety.

Island resorts in the Whitsundays are being evacuated along with low-lying parts of other communities in the danger zone, initially said to reach as far south as Proserpine, near Mackay.

Whitsundays Mayor Mike Brunker told AAP there was a sense of urgency in the community.

"People in low-lying areas are evacuating to friends and family or, if they have to, leave town," Mr Brunker said.

"They should be very anxious as there's no time for complacency."

He said residents had been panic buying food and supplies since Cyclone Anthony, which crossed the coast near Bowen late on Sunday night as a category two storm. It caused little damage.

Yasi expected to make landfall late tomorrow night

Yasi was still well out to sea this morning, but had intensified to a severe category three system.

It's expected to cross the coast as a severe category four, with winds gusting up to 250km/h, late tomorrow night into Thursday morning.

The most likely landfall position is somewhere between Cooktown and Townsville, the Bureau of Meteorology says.

Yasi is expected to deliver a very large storm surge, and the bureau is working to provide emergency services with the latest advice so they can better focus evacuation efforts.

With strong monsoonal activity feeding the storm, it's expected to remain a cyclone long after it makes landfall and could even reach as far inland as Mt Isa as a cyclone with destructive winds, the bureau has warned.

The central Queensland centres of Rockhampton and Emerald, recovering from near record floods in January, are facing further inundation in the days after the cyclone.

Ports from Cairns to Mackay are closing today, along with some schools.

The chances of a storm the strength of Cyclone Yasi hitting New Zealand were "remote", according to Philip Duncan of weatherwatch.co.nz.

"Cyclone Wilma was the first tropical cyclone to hit us at cyclone strength in decades.

"The chances of a New Zealand getting a cyclone of that strength are almost zero. But the chance of getting a storm with similar winds and similar rains is possible."

The long range forecast shows a decreased chance of tropical cyclones striking in the 10 days after Cyclone Yasi hits, Duncan said.

Cyclone may damage Great Barrier Reef

The northern end of the 2600km Great Barrier Reef is directly in the path of Cyclone Yasi.

Capricorn Conservation Council coordinator Michael McCabe said some parts of the reef may die and others could be damaged for decades in the cyclone.

It is already dealing with damage caused by the release of millions of tonnes of sediment during last month's Queensland floods, he said.

"The reef has survived many events in its time but here you have most of the marine life already struggling.

"Coral bleaching can occur in the short term. Entire reefs can become dead skeletons. There are algal blooms. Reefs can take decades to recover, if at all."

Most damage to the Great Barrier Reef from Cyclone Yasi would be caused by flooding releasing more sediment into the sea, said Mr McCabe.

"If you get the right depression you've got potentially another flood situation that would release millions of tonnes of sediment into the ocean.

"Along with that you've got predictions of four metre storm surges and heavy seas that will often cause a lot of physical damage to reefs."

The Great Barrier Reef is the world's largest reef system. It is composed of 2900 individual reefs and stretches over an area of 344,000 square kilometres.

- AAP, AGENCIES, NZ HERALD STAFF