Canberra's water utility now has so much water at its disposal that it has been able to sell some of it back to interstate irrigators and farmers.

ACTEW Corporation's chairman Mark Sullivan told an Assembly committee yesterday that ACTEW was no longer just a purchaser of Murray-Darling Basin water, but had been actively selling in the market since last year.

The territory-owned corporation has bought 12 gigalitres of water across the border and agreed to a deal with the Snowy Hydro Corporation to release flows into the Murrumbidgee River, which could then be extracted at the ACT border, 150 kilometres downstream.

But Mr Sullivan told the multi-party Budget Estimates Committee that with the utility's current surplus of water, it was in a position to sell water to interstate farmers and irrigators.

''Our search for the rainless water option really got down to purchasing water rights in the Murray-Darling Basin, we have a current entitlement of around 12 gigalitres a year,'' Mr Sullivan told the MLAs.

''We're probably one of the first water utilities to activate it, we're getting used to what it is like for a water utility to be an active water trader.'' Mr Sullivan said the corporation was excited to be earning a ''return'' on the entitlements, which are bought on a ''use-it-or-lose-it'' basis.

''After purchasing we are hopeful that we will rarely want the water ourselves, so we're probably party now to establishing a different sort of water rights market,'' he said.

''It's temporary water but we can provide potential purchasers of that water with quite a bit of certainty about when they can get that water, because we will know two or three years in advance if we need to exercise our entitlements over that water.

''So we're a very attractive proposition for someone who doesn't want to hold their capital in water entitlements and are able to spend recurrent funding on water entitlements.''

The water boss expressed his frustration at continued delays to the territory's largest water security project, the Cotter Dam extension, but said the massive construction job still compared well against other Australian dams.

''If you're looking at a water storage project like a dam, you're looking at a decade from policy discussion intents to environmental approval, to construction,'' he said.

''Despite of all of our frustrations in terms of the length of time the Cotter Dam is taking to be constructed, it is one of the fastest dams in this country from a policy decision to a construction point.''