A report written for the Insurance Australia Group, which owns NRMA and CGU, details how releases from Wivenhoe, combined with intense rainfall, flooded hundreds of homes and businesses.
The expert opinion, written by engineering giant WorleyParsons, potentially exposes the State Government to legal action for negligence.
However, the report fails to ascertain how much of the January 13 flood could be attributed to Wivenhoe because Government-owned dam operator SEQWater has refused to detail its water releases in the days prior. The secrecy surrounding the releases has left in limbo victims such as David Stark, who is attempting to write submissions to the flood inquiry.
Mr Stark, whose insurance claim has been rejected by NRMA based on the report, suspects the rapid inundation of his Fairfield home was partly caused by huge volumes of water released by dam operators, in concert with the massive rainfall in other catchments.
"Obviously Wivenhoe contributed, but how much I am not able to be ascertain," he said.
The report by WorleyParsons, which is also a member of the insurance industry's "panel of independent experts" assessing the flood's cause, said its results were based on the information available.
"The elevated levels that were observed in the lower Brisbane River were primarily a response to the intense rainfall that occurred in the upper catchment areas to the west, including heavy rainfall in the headwaters of the Lockyer and Bremer rivers, combined with releases from Wivenhoe Dam," it said.
The report said Ipswich properties near the lower reaches of the Bremer River would have been affected from "backwater flooding from the Brisbane River".
The report detailed how the Brisbane city gauge peaked at 4.5m at 2am on January 13, 30 hours after Wivenhoe operators reportedly released water at a rate of 9000 cubic metres per second to prevent the dam overtopping.
Water discharged from Wivenhoe usually takes about 36 hours to reach this gauge.
Mr Stark said he planned to claim $200,000 in damages because it was obvious the operators should have lowered the dam levels in anticipation of the wet season.