As anxious residents scrambled to build emergency levees and authorities counted billions of dollars in economic damage, researchers said the iconic reef's corals could be damaged by run-off from Australia's eastern coast.

James Cook University researcher Michelle Devlin said floodwaters carrying debris and pesticides spelt a harmful "cocktail" for the world's biggest reef, a delicate ecological treasure and a major tourist attraction.

"This is a really massive event," Devlin told AFP. "It has the potential to shift the food web, it has the potential to shift how the reef operates."

"There is just going to be this cocktail of water containing a lot of things that they (the corals) wouldn't necessarily have seen before," she added. "It is fresh, warm water and that will stress corals out as well."

Devlin said any plume would likely stretch from the reef's southern tip to the scenic Whitsunday Islands, and may in some areas damage sea grass beds -- a feeding ground for dugongs -- or allow damaging crown of thorns starfish to flourish.

Meanwhile, two more military aircraft were pressed into service Wednesday as heavy rains were forecast for flood-damaged areas, which cover an area as big as France and Germany and have affected some 200,000 people.

Queensland premier Anna Bligh said the flooding was unprecedented in the state and had now directly affected 40 towns, raising the number from 22 announced previously.

She said waters that have flooded dozens of mines and closed railways and ports would send coal and steel prices soaring, adding that the state produces about half of the world's coking coal used to make steel.

Queensland Resources Council has said the floods have already cost one billion dollars (one billion US) in delayed coal production, while the state's resources minister says the industry is losing 100 million dollars a day.

"Seventy-five percent of our mines are currently not operating because of this flood, so that's a massive impact on the international markets and the international manufacture of steel," Bligh told the Seven network TV station.

"Without doubt, this disaster is without precedent in its size and its scale here in Queensland," she added.

In Rockhampton, a regional centre of 75,000 that is now an island in a vast inland sea, residents nervously eyed the fast-flowing Fitzroy River, which is expected to peak at 9.4 metres (almost 31 feet).

The town's tourist park was under water, lampposts were submerged up to their lights and waters were lapping at sandbagged stores in the main shopping area.

"To the best of my understanding we have the equivalent of one or two Sydney Harbours of water flowing past Rockhampton on a daily basis," Mayor Brad Carter told reporters, adding that the town could take a year to recover.

"The recovery phase will be long, it will be an extensive process. I couldn't specify the timeframe but it will be several months to six months, possibly a year."

In St. George, which has a history of heavy floods, residents built levees, evacuated nursing home patients and set up a temporary hospital, with waters expected to peak early next week.

"Everyone is pitching in, doing what they can do," said Senator Barnaby Joyce, who lives in the town.

"As soon as this is over we have to concentrate on getting the show back on the road as quickly as possible, otherwise people will be out of work, the price of groceries will go through the roof."

Entire country towns have been evacuated and the floodwaters, at record levels in some places, are now threatening the neighbouring state of New South Wales.

Snakes and marauding crocodiles were among the hazards for the besieged residents of steamy Queensland state, along with disease-carrying mosquitoes and the possibility of looting.

Economists say the disaster could potentially shave about 0.5 percent off Australia's annual GDP, currently running at 2.7 percent.

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