Hundreds of residents have been forced to flee to evacuation centres, with many rescued by boats.

Large areas of Mitchell and Roma are covered by floodwaters, and in Charleville a second flood levee has been built in case the first barriers containing the swollen Warrego River fail.

In the New South Wales north-west, more than a dozen disaster areas have been declared, with 16,500 people stranded.

Water from the bulging Namoi and Darling rivers is expected to be topped up by the Queensland floodwaters and reach as far as Bourke in the days ahead. Queensland Premier Anna Bligh said it looked as though Charleville's levee banks would hold.

''Right now, the levee is holding, and on all current forecasts it's likely to keep doing that,'' she said.

''But there's not very much room for error. The water levels are above the 1997 flood in Charleville.

''So really this is the first test of this infrastructure, and right now it's passing that test.''

More than 600 people have been evacuated throughout the state. Ms Bligh said the people of Charleville were coping with their third major flood in as many years.

''That's too many, and there's a lot of heartache here,'' she said.

Ms Bligh said many towns and properties throughout south-west Queensland had been isolated.

But relief efforts had been boosted by helicopters of the Australian Defence Force.

She said the force's Black Hawk and Chinook helicopters were standing by.

''I'm very pleased the ADF has supplied four helicopters today and we'll have another four tomorrow,'' she said.

''But the people of this region are going to be cut off and isolated for a number of days to come.''

The Insurance Council of Australia has declared catastrophe status for the Queensland floods.

The council said the formal declaration covered the south-west, including the Roma, Mitchell, Charleville and St George regions. Chief executive Rob Whelan said an insurance taskforce had been established.

The New South Wales government said the public cost of the floods had already reached $5 million.

But it said the final amount was expected to far exceed this as more of the state was declared a natural disaster area.