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"The most serious cyclone you can have" is how Queensland Premier Anna Bligh is describing the storm set to ravage the north Queensland coast tonight.

"People in the region from Townsville to Port Douglas need to be psychologically prepared for an event that could last for a terrifying 24 hours", she said.

She warned locals they only had a three-hour window of opportunity remaining to evacuate the area.

The storm threatening to wreak unprecedented havoc on north Queensland has now officially been upgraded to a Category Five cyclone - the most severe level imaginable.

The Australian Board of Meteorology (BoM) this morning issued a warning of what it calls an "extremely dangerous" rise in sea levels on the Queensland coast between Townsville and Port Douglas tonight.

The so-called "storm tide" will occur as Cyclone Yasi approaches and crosses the coastline.

"The sea is likely to steadily rise up to a level which will be very dangerously above the normal tide, with extremely damaging waves, strong currents and flooding of low-lying areas extending some way inland", the BoM advisory says.

The Bureau went on to say the impact of the tropical cyclone is likely to be a "more life threatening event than any experienced in recent generations".

Air New Zealand has cancelled today's flights in and out of Cairns ahead of severe tropical cyclone Yasi.

The airline said that due to poor weather caused by Yasi and the planned closure of Cairns Airport facilities this morning, flights NZ775 Auckland to Cairns and NZ776 Cairns to Auckland are now cancelled.

And is now saying the cyclone is getting more intense by the hour.

"Yasi's air pressure has dramatically fallen to an estimated 922hPa - making it much stronger than Wilma", said WeatherWatch analyst Philip Duncan.

"It now has a clear eye - a sign of a very strong and powerful cyclone."

Meanwhile, tens of thousands of Queenslanders and holidaymakers were last night fleeing their homes or battening down to face what authorities fear will become the biggest storm in the state's recorded history.

Cyclone Yasi was moving rapidly across the Pacific from Vanuatu and was forecast to directly strike Cairns and the far north coast shortly after midnight tonight (NZT), although conditions are likely to become potentially lethal from this morning.

With Queensland still reeling from flooding, Premier Anna Bligh warned: "This storm is huge and it is life-threatening."

But as extra flights raced to evacuate residents, hospitals and tourists from mainland towns and Whitsunday resort islands before airports closed, Ms Bligh said the state was "not battle-weary - we're battle hardened".

Yasi is now expected to hit the coast as a category five cyclone, larger and more intense than Cyclone Larry, which hammered far north Australia in March 2006, taking no lives but causing damage estimated at A$1.5 billion ($1.8 billion).

Winds at their worst could slam into coastal communities at up to 280km/h, weakening towards the cyclone's outer rim - but still dangerous and destructive between Cooktown and Townsville - and possibly affecting towns as far inland as Mt Isa, 900km from the coast.

The cyclone's fury will also drive storm surges, threatening lives and homes in low-lying areas, especially in the Cairns region between Mossman and Cardwell, where police were yesterday door-knocking and advising people to leave.

About 250,000 people live in the area most at risk, and police urged residents to leave voluntarily to avoid mandatory evacuation orders and the possibility of late mass movements that could threaten lives and stretch emergency resources.

Ms Bligh said international research showed that more people died from storm surges than cyclonic winds. Homes could be left without power or water for up to five days. Many roads are likely to be cut or extremely dangerous from this morning.

"I think many people will be very frightened by what they're hearing," Ms Bligh said. "I don't want to frighten people, or panic them, but all the information I'm getting is that we are facing a potentially very deadly event. It will be a display of the awesome power of nature but it's not something you want to go outside and watch."

Cairns-based New Zealander Rush Pathak said last night residents were literally "battening down the hatches" and stocking up on water, food and petrol.

"Right now it's eerily calm here, like the calm before a storm, as everyone prepares for this massive mammoth cyclone. I was at the supermarket before and it's completely packed with people stocking up. And the lines at the fill-up stations are out the door. It's kind of like a mass hysteria of people preparing themselves."

Although buildings have been designed to resist the power of cyclones since Cyclone Tracy flattened Darwin in 1974, many older constructions remain highly vulnerable.

Cairns area emergency services chief Wayne Hepple said Yasi's surge would strike south from the cyclone's eye. "If you are on that southern side, that's why this thing is going to be quite dangerous," he said.

Yesterday afternoon about 250 patients from the seafront Cairns Base Hospital and another private hospital were flown to Brisbane by RAAF Hercules, believed to be the largest single medical evacuation in Queensland's history.

Hospitals in Brisbane were placed on "brown alert", requiring them to cancel elective surgery and send other patients home early if emergency beds are needed.

Winds could be too dangerous to move outside buildings by as early as 8am today.

Evacuation of low-lying areas in Cairns began yesterday, seaside retirement and nursing homes in Townsville were emptied, and Mackay was preparing for a surge as the area's tide rose higher than normal.

As Qantas, Jetstar and Virgin flew extra aircraft into the target region yesterday to help evacuate people, all far north schools and universities were closed until Friday.

All ports from Cairns to Mackay have been closed, and a yellow alert - requiring vessels to move to safe anchorages or be hauled from the water - has been issued by Maritime Safety Queensland for all boats from Cape Flattery in Cape York to Mourilyan, south of Innisfail.

Near Brisbane, dam managers were preparing to release water from the big Wivenhoe flood mitigation dam if Yasi dumps more torrential rain across the already flood-stricken region.

280km/h Wind speeds expected to strike mainland

250,000 Residents living in most at-risk areas

250 Patients moved from Cairns hospitals

5 Cyclone category rating - went up from 4 this morning.

- Additional reporting by AAP, Greg Ansley, Amelia Wade and nzherald staff