The ACT Opposition says it is no closer to getting detailed answers on the Cotter Dam cost blow-out despite a lengthy briefing from Canberra's water authority ACTEW.

Liberal MLAs met with ACTEW on Monday about the cost of increasing the Cotter Dam's capacity to 77 gigalitres.

In September ACTEW revealed the cost of the project has increased by another $117 million to $363 million. It was originally estimated in 2005 that it would cost $120 million.

Last week ACTEW supplied cost estimate documents to the Legislative Assembly.

The Opposition complained the documents were too heavily censored and provided little information.

Leader Zed Seselja says the meeting with ACTEW has not shed anymore light on the situation.

"We simply don't know where the bill for this project will end," he said.

"We've been assured that $363 million is the end of the cost blow-outs - we simply have no way of knowing. We don't have the detail that would assure us of that.

"The process to date has not been a good one and so we have real concerns.

"But the cost blow-out to date has been enough of a concern and we need answers and we still don't have those answers."

Mr Seselja says taxpayers need to be told the whole story.

"The answer that it's all commercial-in-confidence is not acceptable," he said.

"It's a Territory owned corporation, the Government can't hide behind commercial-in-confidence when we're talking about a quarter-of-a-billion dollar cost blow-out.

"Members of the Assembly on behalf of the community have a right to know so that we can determine what went wrong and so we can try and figure out a way to ensure these kinds of cost blow-outs don't occur in the future."

The Opposition has been calling for a Legislative Assembly inquiry into the cost blow-out but has been unable to secure enough support.

ACTEW is giving a briefing to Greens MLAs this morning.

Last week Greens MLA Shane Rattenbury said his party would wait until after the meeting before making a decision on whether or not to support an inquiry.
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