THE equivalent of a year's supply of drinking water will be released from southeast Queensland's Wivenhoe Dam with further heavy rain forecast for the flooded region.

The dam's role in last month's flooding of Brisbane remains controversial, with some critics claiming that earlier releases of water would have stopped the inundation of up to 20,000 properties.

It is one of the topics to be probed by a commission of inquiry this year.

From next weekend, 25 per cent of the dam's water will be released slowly over nine days, adjusted with rainfall and the tides.

Announcing the massive release today, authorities stressed no homes or farms would be flooded.

Natural Resources Minister Stephen Robertson said the 290,000 megalitre release was needed to make space in the dam to mitigate any further flooding, with heavy rain forecast until April.

The second strongest La Nina weather pattern in history prevailing over Queensland made the "very conservative and precautionary" approach necessary, Mr Robertson said.

"The Brisbane River will not break its banks as a result of these releases, and there will be no significant downstream impacts," he told reporters in Brisbane.

Seqwater chief executive Peter Borrows said it was too difficult to say whether a 25 per cent release would stop another flood on the scale of those seen in January - but guessed it was "extremely unlikely".

"It would have had to have been a major reduction in the storage to have any impact on a major flood event," he said.

Mr Borrows stood by the management of the dam, saying it went by the book in the face of unprecedented flooding.

Some 2.6 million megalitres of water flowed into the Somerset-Wivenhoe system in January's floods, more than double that of 1974.

SEQ Water Grid chief Barry Dennien said although the release was equivalent to a year's normal drinking water supply, above average rainfall would mean it would be recovered more easily.

In addition, the Wyaralong Dam was now full, storing 103,000 megalitres of drinking water five years earlier than expected.

"Our (water) security position is very sound," he said.

The dam release will have no effect on water bills or restrictions, but will close three bridges downstream of Wivenhoe - Twin Bridges, Colleges Crossing and Savages Crossing - for at least nine days.

Emptying the dam to 75 per cent capacity will be a temporary measure, with its future operation to be informed by the floods inquiry.