The ACT Independent Pricing and Regulatory Commission (ICRC) has announced annual water and wastewater bills will increase by nearly 12 per cent from July, or about $142.


The regulator says the cost of long-term water security projects like the Cotter Dam enlargement, and a reduced demand for water are behind the price rise.


ACTEW has already spent $33 million buying the rights to water from Tantangara reservoir.


Chief executive Mark Sullivan says protecting the capital's long-term water supply is vital.


"Every examination of climate change scenarios that we have done says that if we don't do Tantangara, we don't do Cotter Dam and we don't do the planned pipeline, then Canberrans will again face water restrictions in the short term," he said.


"If [the ICRC] had got the water volumes right in 2008 we would have been paying this increased price from then. But the regulator modelled a higher volume of water - which pushed the price down," he said.


"We're now seeing an adjustment as a result of those water volumes not occurring."


The ACT Opposition says the increase is the result of mismanagement of water infrastructure projects like the Cotter Dam enlargement.


But Chief Minister Jon Stanhope has rejected the claims.


"We're looking to the future, we're providing security for the future, the people of Canberra want it, they applaud it, but it comes at a price," he said.


"It was always going to come at a price, infrastructure costing in excess of half a billion dollars has to be paid for, it has to be paid for over the term and of course it was always going to come back to the price of water."