Historically large floods could only take some of the blame and just $22 million had been set aside for contingencies for the highly complex project, territory auditor-general Maxine Cooper said in her report released Wednesday.

The final cost of the dam enlargement reached $410.5 million, $47.5 million more than originally expected.

The dam project included a new, 80 metres high and 240 metres wide dam wall about 100 metres downstream from the old dam wall, as well as two substantial earth embankment saddle dams adjacent to the main dam, the upgrade of nearby recreational facilities and the construction of seven kilometres of artificial fish habitat.

Factors contributing to the increased time and costs of the project included a previously undetected geological fault at the base of the abutment of the dam - described as "not a reasonably foreseeable risk" - and work to plan for and avoid flood damage at the site, some of which also could not be predicted.

"While a decision was made comparatively early in the construction phase of the project to change the scope of the project and increase the project's flood mitigation diversion capacity, the diversion was ultimately inadequate in dealing with the February/March 2012 flood event," the report said.

"However, the February/March 2012 flood event was a 1 in 100 year flood.

"The costs associated with preparing a diversion with the capacity to manage and mitigate a 1 in 100 year flood event would have been very high and unwarranted from a risk management perspective."

Other delays were forseeable, such as the time it took to excavate and clean the foundations of the dam in preparation for the placement of the dam wall and slower than anticipated placement of roller compacted concrete in the dam wall.

The report said the expectations of the designers were poorly understood by the constructors at the stage when the project's estimated cost was being calculated.

In conclusion Dr Cooper said: "The construction of the enlarged Cotter Dam was a high value, complex and long-term project and as such ACTEW's [now Icon's] use of an alliance to deliver it and the Murrumbidgee to Googong pipeline was appropriate and effective."

Icon Water and the project contractors who formed the alliance with the dam operator - GHD, Lend Lease and John Holland - responded to the report findings by saying the dam enlargement was one of the most significant infrastructure projects in Canberra's history.

It had been done in the context of the booming mining and resources sectors, the global financial crisis and a severe drought uniquely coupled with two of the wettest years in Canberra's recorded history.

"Its construction required unique skills and practices, 24-hour-a-day production, careful consideration of environmental requirements and continuous management of the high level of risk to personal safety associated with the work," the response said.

"During construction a total of 2.5 million hours were worked by over 4000 project personnel working in temperatures ranging from ‐8 to 40 degrees.

"Their work was rain‐affected on 246 days and their worksite was devastated by an extraordinary flood event in 2012, which followed smaller floods in 2010 and 2011."

Meanwhile, two other projects done under the same alliance registered comparatively better results.

The Murrumbidgee to Googong Pipeline cost $140.5 million, less than the final estimated cost of $154.5 million and was overdue by three months. And the Googong Dam Spillway cost $54.0 million, less than the final estimated cost of $56.0 million and was overdue by eight months.

The combined cost of the three projects cost $605 million, an increase of 5.5 per cent over the combined final budgeted costs.

ACT opposition infrastructure spokesman Alistair Coe said the report seemed to show a lack of formal communication between the water utility and territory government ministers which suggested Icon "was either operating independent of the ministry, or the communication was informal".