The Insurance Council of Australia report concludes that releases from Wivenhoe Dam were the "principal immediate cause" of flooding along the Brisbane River, Lockyer Creek and Bremer River downstream.
The report, published this week, states releases from the dam increased water levels by the following:
- 5m at the Lockyer Creek at O'Reillys Weir near Lowood
- More than 7m in the lower Bremer River at One Mile Bridge
- 7m in the Brisbane River at Savages Crossing and 10m at Mt Crosby
But the report, prepared by an independent panel from Worley Parsons, WRM and Water Matters International, said dam operators could not be blamed for the flood event with a number of contributing factors playing a part.
The panel was not able to work out what contributions the Lockyer Creek and Bremer River added to the Brisbane River's flooding.
An Australian rivers expert said the report's estimates of how extra water the dam releases added to flood waters in Ipswich was close to what he expected.
Professor Andrew Dragun, an adjunct professor for Griffith University's Australian Rivers Institute, said those increases to flood levels could have been avoided if releases were made from the Wivenhoe and Somerset Dams earlier.
"There wouldn't have been an extra flooding at all," he said.
The report urges insurers not to decline claims based on the definition of a flood, and said some inundated properties may have been hit initially by flash-flooding caused by heavy rain.
It said some properties that were inundated, further away from the river, may have been hit initially by flash-flooding caused by heavy rain.
Earlier this week the Wivenhoe Dam's operator, Seqwater, released a report on its own investigation into the flooding, which blamed weather forecasters for bad predictions. Their self-assessment found the releases from the dam did not significantly contribute to the January floods.
What part the dam operators played in the floods will be officially looked at by the upcoming Queensland Floods Commission of Inquiry, due to start next month.
The commission's first report is due on August 17 and its final report is expected in January 2012.
Public hearings have been scheduled for Ipswich on May 20.
More flood news: www.QT.com.au