The 14,690ML figure provided by ACTEW Water does not include the 21,466ML released from Googong Dam between September 18 and October 9 last year, which would have taken the total since September 2013 to more than 36,000ML.

Adam MacLaren, a bricklayer from Banks who fishes for redfin on Googong Dam several times a week, said strong winds had forced water over the spillway last week with the dam at capacity.

"It's been pretty full for the last five years and it's not uncommon to see water spilling over after it rains," he said.

Deputy Opposition Leader Alistair Coe said the government had missed an opportunity to improve the ACT's water security heading into summer, despite storage levels being the highest in years.

"The government's key argument for the need to construct the enlarged Cotter Dam was water security. However the management of the now constructed dam does not seem to reflect that need," he said.

"Given Googong is at capacity and the Cotter is at 85 per cent, it makes sense to use water from Googong to avoid losing water over the Googong spillway and allow the Cotter Dam to fill up at a faster rate due to water not being extracted."

But despite the opposition's concerns, an ACTEW Water spokeswoman said Canberra's water supply was healthier than ever thanks to significant investments during the past decade.

"Canberra's catchments are currently sitting at 86 per cent full, which represents 238 gigalitres of water – more water than we've ever had before," she said.

ACTEW Water has advised the government it would have cost an additional estimated minimum of $3.8 million to source water exclusively from the Googong Dam over the past year.

"When we operate the water supply system we have two competing objectives: to maintain water security as high as possible [and to] supply water to the community at the lowest possible cost," the spokeswoman said.

"It isn't possible to always maximise both of these objectives."

But Mr Coe said the financial worth of water lost over the Googong Dam spillway was far greater than the cost of supplying Canberra with water from the dam.

"If the lost water was still in the system, we would be going into summer and possibly a drought with 100 per cent water supply and the value of saved water would far exceed $3.8 million," he said.

"If water had been drawn from the Cotter Dam rather than Googong, it's likely our storage would be at 100 per cent rather than the present 85 per cent."

But the ACTEW Water spokeswoman said comparing the value of water lost from the dam to the cost of supplying water was not reasonable.

"The amount of water that has been consumed in the last 12 months (and into the future) wouldn't change if we chose to and were able to manage our system so that Googong didn't spill," she said.

The spokeswoman said it was more expensive to use water from Googong Dam (663m ASL) because it was at a lower elevation than Bendora (778m ASL) and Stromlo (721m ASL) and would require water to be pumped rather than gravity fed.

"It is important that when operating our system, we are achieving the most efficient method of water supply for the community, whilst minimising the impact on the storage capacity overall," she said.

Queanbeyan City Council has routinely called for Googong Dam to be capped at 85 per cent capacity to create an "air reserve", which would serve as a buffer against any repeat of the 2010 flood, which caused $1.3 million of damage.

But ACTEW Water said keeping water levels at 80 per cent to 85 per cent of capacity would have done little to stop the 2010 flood, despite its agreeing to take part in an independent study with Queanbeyan council to consider other ways to mitigate flooding.

"The results indicate there is very little security benefit in running Googong Dam when storages are high, as they are at present," the spokeswoman said.

But Mr Coe said "numerous constituents" had continued to raise concerns about management of Canberra's water supply, including flood management.

"There is no doubt that Googong Dam could be better utilised for flood mitigation for Queanbeyan compared to the present management of the dam," he said.

"The risk to life and property of the Cotter Dam being at 100 per cent is considerably less than the risk to life and property when Googong is at 100 per cent."

Googong Dam provided 19 per cent of the water supplied to Canberra and Queanbeyan during 2013-14.