The Greens will table a motion in the Legislative Assembly next week for the blow-out to be assessed by the Independent Competition and Regulatory Commission, but it needs to win the backing of the Liberals.

The Greens and the Liberals received a briefing this week from Actew on the cost increase, but Greens environment spokesman Shane Rattenbury said it lacked detail.

''I think there are questions remaining and at some point the ACT consumer has to pay for this dam and we want to have the most qualified people to access whether this cost is justifiable,'' Mr Rattenbury said.

Opposition leader Zed Seselja said he would consider the Greens motion.

Actew issued a revised estimate of $363 million for the project in September, three times initial predictions of $120 million in 2005.

An independent review conducted by accounting firm Deloittes found the cost estimates for the dam enlargement lacked transparency, had inadequate contingencies and that poor communication contributed to the predicted cost tripling.

The Greens and the Liberals both raised concerns about the level of censorship in the documents.

When completed, the Cotter Dam expansion will increase the dam's capacity from 4 to 78 gigalitres, and total ACT storage by 35 per cent.

For more details on this story, pick up a copy of today's Canberra Times.