Residents of Rockhampton, Australia, joined a growing number of evacuees on Saturday as floodwaters threatened to sink the coastal city as the water spread across the country.

Residents fled rising waters on a dark Saturday night as helicopters dropped off supplies from the now-isolated city.

Days of rain have overflowed riverbeds and flooded an area of Australia larger than France and Germany combined.

"In many ways, it is a disaster of biblical proportions," Queensland Treasurer Andrew Fraser told reporters in the flooded city of Bundaberg on Saturday.

So far, around 200,000 people have been affected by the floodwaters as the water spreads and makes its way toward the ocean.

"We've never seen flooding like this," Greg Goebel, director of the Australian Red Cross told CTV News Channel from Brisbane. "There is this tide of water that is simply engulfing a number of towns. In fact, whole towns have been evacuated, some of them at night time.

Residents in Rockhampton, a town of 75,000 that is considered the beef capital of Australia, stocked up on food and supplies on Saturday as officials prepared to evacuate those living in low-lying areas to an evacuation centre.

Goebel said the floodwaters are expected to flow right through the northern Bowen Basin, the second-largest water catchments area in the country.

"The water is going to cut off all road transport and the airport is closed and that is predicted to happen for about three weeks," he said. "That town has to a very difficult time ahead."

Rockhampton Mayor Brad Carter told The Associated Press that about 40 per cent of the city could be consumed by the flood, which thousands of homes at risk.

"Some of them will not know whether their floorboards have been covered and their personal property destroyed, or whether they've been saved and the water has only come up and spared their property," Carter said. "That's going to be a difficult waiting period for many members of our community."

Earlier in the week, the military was called to clear several towns in the eastern state of Queensland, where now more than half of its 1.8 million square kilometres has been affected by the flood.

Queensland Premier Anna Bligh has said that some communities could be underwater for more than a week.

Cleanup costs are expected to reach into the billions of dollars.

Goebel said the government's response to the crisis has been very coordinated and organized so far as it worked with the military and Red Cross to clear the flooded plains.

He said a relief fund is being organized that will go toward paying for the emergency relief and assisting those displaced by the flooding.

"The real issue is that many of them will not get home for days, maybe weeks. We've had some people that have really become refugees in this country. They have been uplifted by helicopter, moved to a town hundreds of kilometres away and not likely to get home for some weeks," Goebel said.