Severe Tropical Cyclone Yasi has been upgraded to a category four system as it moves steadily towards north Queensland, where tens of thousands of people are fleeing their homes.

The Bureau of Meteorology says Yasi is still about 800 kilometres off the coast with wind gusts of up to 260 kilometres per hour at its centre.

The Tropical Cyclone Warning Centre says damaging wind gusts of up to 90 kilometres per hour are expected on coastal islands this morning.

It says there will be damaging gusts in excess of 125 kilometres per hour between Cooktown and Ingham by this afternoon.

Yasi is still on track to cross the coast somewhere between Cairns and Innisfail about 1am AEST on Thursday morning.

Its destructive winds and associated storm surges could affect an area from Cape Melville to Sarina.

The cyclone warning also extends inland to Georgetown and west of Charters Towers.

Residents in low-lying parts of Cairns and Townsville have been told to move to higher ground and patients from two Cairns hospitals are being flown to Brisbane overnight.

The Mayor of Cairns says the town is expecting damaging winds from Yasi to hit the town from midday today.

Mayor Val Schier has told Lateline that a storm surge close to three metres high is expected to arrive well before the cyclone hits.

She says far north Queensland region is facing its biggest cyclone in living memory.

"That obviously is going to inundate quite a lot of houses," she said.

"We've actually asked more than 30,000 people to evacuate; that's all those low-lying areas from Port Douglas, the northern beaches of Cairns, and right down to Bramston beach near Innisfail."

She says people should leave for higher ground or seek shelter at evacuation centres, which open at 6am today.

People who have to leave their homes and have nowhere to go should call 1300 99 31 91 for help finding emergency accommodation.

Ms Schier says the town is still recovering from an economic slow-down in recent years.

"We were just getting back on our feet and feeling really confident in the tourism industry and in our other industries as well, so this will be a big set-back if we end up getting a lot of damage as is predicted at the moment," she said.

Cairns resident Melissa Lovejoy is among the thousands in low-lying areas fleeing the cyclone.

"We just received a phone call and a text message that advised those in coastal low-lying areas, which is exactly where we're living, to get out tonight," she said.

"So we're in the process of packing up boxes with essential and family heirloom-type things, the dogs and the pet snake and getting out of here."

She says it has been an emotional experience packing up her house.

"I keep telling myself if we lose this house, cause we're totally on the water, that it's just stuff, it can all be replaced. But sometimes your heart doesn't go that way," she said.

"And also just not knowing how much damage it's going to do and scared for other people, scared for the wildlife, and scared for Cairns; it's such a beautiful region. I'd hate to see it decimated by such severe winds and rain."

Another Cairns resident Graham Burridge says he expects his house will be destroyed by the predicted storm surge.

"We're eight metres away from the water and if the predictions are right about the storm surge, then if we stayed here we'd be using the house as a surf board and probably end up 20 kilometres inland," he said.

"So we've just decided discretion is the best part of valour and we'd rather see the house go ta ta and take our animals and valuables with us and go somewhere safe."

Army called to help

Meanwhile, soldiers are about to start doorknocking homes in low-lying areas of Townsville and issuing mandatory evacuation notices.

Queensland Premier Anna Bligh ordered the mandatory evacuations after lengthy discussions with the Townsville City Council yesterday afternoon about the threat from a cyclonic storm surge.

About 15,000 people have been ordered to leave almost 4,000 homes across the city.

Emergency evacuation centres have been activated to cater for up to 10,000 people.

Councillor Dale Last says forecasters expect winds of more than 100 kilometres per hour this morning and the evacuations have to be done beforehand.

"The level we're talking about is 5.1 metres, the normal king tide in Townsville is 4.1 metres ... so a metre above that," he said.

"So if you live in one of those low-lying areas, such as South Townsville, Railway Estate, you need to work out just where that extra one metre of water will reach, hence the reason why we're mandatory evacuating.

Council is hoping to have moved everyone out of the affected suburbs by the morning.

"The issue we have and the reason we're undertaking it tonight or starting to do the doorknock tonight is that we're expecting gale force building into destructive winds commencing from mid morning tomorrow," Councillor Dale said.

"So we just won't have the time tomorrow to go and doorknock the number of homes that we need to do so that's the reason we're starting tonight."

South of Cairns, cyclone preparations are well underway in the town of Cardwell.

The weather bureau predicts it will be subject to winds in excess of 125 kilometres per hour from this afternoon - 12 hours before Yasi crosses the coast.

Cardwell residents spent yesterday boarding up windows and stocking up on essentials.

Patrick Quirk from Maritime Safety Queensland says commercial shipping and recreational boat users have been well warned over the past week and should know to stay away from Cyclone Yasi.

He says he hopes no-one at sea is foolish enough to get too close to the system moving rapidly across the Coral Sea.

"There were some pictures going around comparing this to Larry, but this is a system which is almost beyond belief," he said.

"It's almost beyond description in the extent of the system and also the power of the system. It's almost a perfect storm."