The ACT Greens had planned to move a motion in the Legislative Assembly yesterday asking the Government to refer an inquiry to the commission, but did not need to after Environment Minister Simon Corbell agreed to do so.

The two parties agreed on terms of reference last night which Mr Corbell will issue commissioner Paul Baxter with today. They order him to report on whether ''the projected costs of the enlarged Cotter Dam water security project are prudent and efficient'', the potential for any further cost variations, and the scope for savings.

The ACT Government-owned water-provider Actew Corporation had said in 2005 that the project would cost $145 million, but after multiple revisions the price tag now stands at $363 million.

Mr Corbell told The Canberra Times yesterday the commissioner was the best body to review the matter because of his expertise in examining such things.

''It allows us and the Assembly to get an independent and expert review of the cost estimates for the project and whether they're prudent and efficient,'' Mr Corbell said.

''I think that will give the Assembly a very detailed and expert analysis of the project.''

Mr Corbell will ask Mr Baxter to report by the end of June 2010.

Greens environment spokesman Shane Rattenbury said he was pleased with the terms of reference.

Although he had favoured asking Mr Baxter to perform a price determination which would have required a full review of all aspects of Actew's operation, not just Cotter Dam he believed the terms were sufficiently broad so as to provide the answers he wanted.

''I am happy that the commissioner has the latitude to make the determinations that he needs to make to deliver the information that the Canberra community needs,'' Mr Rattenbury said.

But Opposition leader Zed Seselja, who had unsuccessfully sought to have the blow-out referred to a Legislative Assembly committee, said the terms of reference would not hold the relevant Government ministers accountable.