ACTEW managing director Mark Sullivan told a Legislative Assembly committee that the territory's dams were between 65 and 70 per cent full. "At 40 per cent full, the new Cotter is now eight times the size of the old Cotter reservoir, and it's tracking well and truly beyond expectations,'' Mr Sullivan said.

He said a hydrologist had predicted it would take another two to three years for the Cotter Dam to fill but he believed this may occur more quickly.

The inquiry also heard that the final cost of the new dam would probably not be known until late next year. The project has been hit by repeated cost blowouts.

Mr Sullivan said he was confident that the final cost would be within the $409 million range.

"I probably have more confidence today than ever that it will probably not exceed the $409 million. I think it will be somewhere in that range.''

Insurance payouts would have an impact on the final cost to ACTEW.

The ACT will be given control of water from the smaller Googong Dam under legislation expected to be passed by federal Parliament on Thursday. The surface water of the dam is to be treated as if it were located in the ACT.

The change will allow the ACT to include all of the territory's water resources in its water resource plan, as is required under the Murray-Darling Basin plan.

The change will not affect any agreements for the supply of water to Queanbeyan.

The Googong Dam was constructed in the 1970s on Commonwealth land on the Queanbeyan River with the water reserved for use by the ACT.

In 2008 the ACT signed a 150-year lease over the land.

Legislation will also make the ACT government responsible for managing water on national land in the ACT, which is now a federal function.