Qualified approval has been granted to the ACT's enlarged Cotter Dam, the Murrumbidgee pipeline and spillway upgrade plans, despite auditors spotting foreseeable delays and a cost blowout.

The three major infrastructure projects were first announced in 2007 as a response to the major drought in Canberra between 2002 and 2010.

The aim of the three major infrastructure projects was to improve water security for the ACT.

ACT Auditor-General Maxine Cooper has released a report finding that ACTEW’s use of a contracting alliance for the projects was appropriate and effective.

The ACT’s Bulk Water Alliance consists of authorities from ACTEW, the project designer GHD and builders from Abigroup and John Holland Group.

The finding comes despite the fact that the Cotter Dam project ended up costing $410 million, considerably more than the estimated figure of $363 million.

The auditors were looking for a reason behind the 20-month delay and cost overrun, and said that events such as flooding and a geographical fault did not fully account for it.

The auditor general says delays in concreting and excavation should have been foreseeable, but did not find that delays in providing project costs to the public were deliberate.

“There is no documented evidence that ACTEW or the ACT Government sought to deliberately mislead or deceive the public,” Ms Cooper said in the report.

The report also found no merit for including the Googong Dam Spillway project in the alliance.