A report into last year's devastating Australian floods found Friday that dam engineers breached their operating manual as the crisis deepened by delaying the increase of water releases.

The 2011 Queensland floods are likely the most costly natural disaster in Australian history, swamping an area larger than France and Germany combined, flooding 29,000 homes and businesses and affecting 2.5 million people.

Thirty-three people died in the disaster, which is estimated to have cost more than Aus$5 billion (US$5.3 billion) and which flooded the Brisbane River, bringing the Queensland capital to a standstill.

In its 658-page report, the Queensland Floods Commission of Inquiry found that Brisbane's Wivenhoe Dam was operated in breach of its manual from early on January 8 until late on January 9, before Brisbane and nearby Ipswich flooded.

"What should not be overlooked is that the manual itself was ambiguous, unclear and difficult to use, and was not based on the best, most current research and information," it said.

Despite the breach, expert evidence to the inquiry was that the flood engineers had managed Wivenhoe so that its flood mitigation effect was very close to the maximum achievable within the constraints of the manual, it added.

The report recommended that three engineers working at Wivenhoe during the crisis be referred to the Crimes and Misconduct Commission over whether they had misled the inquiry. The three deny any cover up.

Queensland Premier Anna Bligh acknowledged the prospect of a class action against Seqwater, the government-owned operator of the dam, was now possible but said the report on its own did not prove liability.

"It opens the possibility of legal action but it does not, of itself, establish liability," she said.

In its report, the commission said while there was no doubt the floods took a state more accustomed to drought by surprise, officials had responded well.

The response, if not perfect, "compares favourably with the apparent paralysis of government agencies and breakdown in order apparent on the Gulf coast after Hurricane Katrina struck New Orleans", it said.

But it noted room for improvement in planning, despite the fact the floods were unprecedented, at times completely unexpected, and striking at so many points at once that no government could have met all demand.

The report said that while the Wivenhoe Dam had a flood mitigation capacity, this was reduced by the volume of water it was holding ahead of the downpours which began in December 2010.

It said while the amount of rain due to fall into the dam could not have been predicted, what was concerning was the "apparent inertia of government when the possibility was raised".