The enlarged dam now holds 76 gigalitres (GL), nearly 20 times its original size, and increases the ACT's storage capacity by 35 per cent. The dam is 99.68 per cent full and expected to be running over the spillway on Thursday.

Although the Cotter is holding more water than ever, the ACT's storage is slightly behind the 2014 record of 86.8 per cent, because Corin Dam is holding 41 per cent less water than it did in 2014.

Icon managing director John Knox says Canberra's water storage is healthy and brimming.

"When we celebrated the completion and opening of the dam in 2013, it was already nearly a third full of water thanks to the better than anticipated rainfall and inflows of 2012," Mr Knox said.

The $409 million extension employed 3000 workers over four years, when the ACT received its heaviest rain in recorded history.

"Today's declaration of a full dam is a great day for Canberra's water story and bodes well for future water security for our community, as our population and city grows, and our region expands," Mr Knox said.

Googong Dam which supplies 43 per cent of Canberra's supply, has 119.41GL capacity and is 100 per cent full.

Icon Water is confident water restrictions will not be required in Canberra and Queanbeyan for many years unless there is a worse drought than has occurred in 145 years of recorded data.

Bendora Dam (8.03GL) is 70.16 per cent full.

Water is spilling over the top of the Googong Dam. "We have an environmental release requirement of 10ML per day and we are releasing 80ML per day through our two hydros to generate power while the dam is at capacity," a spokeswoman said.

"There is additional overflow from the dam of approximately 130ML per day.

"Similarly, once Cotter starts to spill, we are unable to stop it from spilling until the in-flows stop flowing into the catchment. Water is being released at our required environmental flow release rate of 35ML per day," the spokeswoman said.

Icon says Googong was not designed to prevent downstream flooding, even though the dam on the Queanbeyan River helps mitigate downstream flooding.

"Icon Water has no particular role in a flood event, other than monitoring how the dam is working during the event," a spokeswoman said. "All other flood related operations are undertaken by the Bureau of Metrology and NSW State Emergency Service."

Storages have bounced back since April , when the ACT's four dams had fallen to about 75 per cent full, compared with 80 per cent last year. More water is on the way, according to the Bureau of Meteorology.

From Sunday to Tuesday, between 10mm and 25mm is expected, most of it on Monday.

BOM senior climatologist Agata Imielska says said the outlook from now until September favours wetter conditions, and an 80 per cent chance of above average falls.

Murrumbidgee Golf Course general manager Sue Terry says the course's three dams are full, a welcome sight for any course these days.

"A golf course doesn't take that long to dry out and evaporation can take that water," Ms Terry said. The club had previously had to buy water, but in recent years rainfall had saved the expense.