The Bureau of Meteorology is already predicting a drier than average winter across eastern and southern Australia this year.

Icon Water senior analytics engineer Timothy Purves said Canberra's water restrictions began when dam levels reached 35 to 40 per cent.

Canberra's collective dam levels currently sit at about 57 per cent.

"It would be a possibility some time in 2020 if the drought doesn't break between now and then," Mr Purves said.

"It would have to be a diabolically bad year for storage to go down between now and the end of the year."

Sydneysiders are set to face their first water restrictions in a decade on Saturday, with the city's dams hitting the "trigger" of about 53 per cent.

Mr Purves said thanks to Cotter Dam, Canberrans had a lot more water storage than most other cities.

Canberra's trigger of about 35 to 40 per cent was equivalent to the dam levels being at 50 per cent before the Cotter Dam was built, Mr Purves said.

Environment commissioner Professor Kate Auty, who recently reported on the poor state of water protection and resourcing in the ACT, said Canberrans needed to rethink how they used water.

"We really need to rethink things like our gardens. We need to concentrate on things like how we'e going to store water ourselves," Professor Auty said.

"It's not going to get any better."

Mr Purves said the water utility had modelled for every drought that could ever happen and even then couldn't run out of water.

He said it would review their plans when dam levels hit about 50 per cent.

"Our consumption levels have decreased significantly over the past 20 years," Mr Purves said.

"In fact we asked the community to save a whole lot of water during the millennium drought, which they did. We've seen a slight increase since."

He said while Canberrans weren't under water restrictions yet, there were still measures in places, like bans on watering gardens during daylight hours in the summer.

"There are actually quite a number of measures in place ... most of them are pretty much common sense things," he said.

Canberra's largest dam, Googong, is about 61 per cent full. The Cotter, the capital's second-largest, is about 83 per cent full. The smallest, Bendora Dam, is about 50 per cent full.

The Corin Dam, the third-largest, is about 23 per cent full but Mr Purves said this was normal.

Mr Purves said it was Icon Water's first source with a highly reliable quality of water and because it mainly pumped downhill, also one of the cheapest to use.

"Our next preference is Googong dam. The Cotter Dam is the last of the dams that we go to," Mr Purves said.