Since 2016, Canberra's once-healthy water storage levels have been in steep decline. Out at Googong Dam, the telltale earthen line which separates the vegetation from the water level is widening as the cloudless days lengthen.

In June last year, the storage held within the ACT's four major dams was at 69 per cent capacity. In less than 18 months it has fallen to 54 per cent and heading drainward.

Troubling for Icon Water, which manages our water storage and supply, is that demand for the precious resource from a growing Canberra population is galloping ahead of supply.

Inflows to Canberra's dams, measured over comparative 24-month periods, are running at their lowest since the "millenium drought" from 1997 to 2010.

If this weather pattern continues, Icon Water says the return of water restrictions for Canberra and Queanbeyan are less than a year away.

For those dismissive of the looming shortages and wave it away as vagaries of everyday weather, the lessons of the past are well worth observing.

Rain was falling, Canberra's outfields were lush and green, dam levels healthy and the future untroubled when a commitment of $360 million - later to blow out by $50 million - was made to the enlargement of the Cotter Dam.

Mid-construction, such was the volume of water inflow at the time from a 1-in-100 year flood, the dam overflowed and equipment was washed away.

"I was on the dam wall on a two-way radio at the time, watching it happen; it was pretty nervous time," Icon Water's managing director Ray Hezkial recalled.

Speculative eyebrows were raised as to whether the public money might be better spent elsewhere but engineering foresight prevailed, the rain stopped, and the decision long since vindicated. It was an infrastructure masterstroke which restored a buffer zone to the city's water security.

Without it, the city today would be under water restrictions.

Regionally surrounded by drought and dry dams, Canberra has been an oasis.

But for how long? Rainfall for the nine months to the end of to the September 2019 was 308mm, and the Bureau of Meteorology expects the dry outlook to "continue into December".

Even contingency engineering measures put in place after the previous drought have dried up.

The Murrumbidgee River to Googong water transfer is a pipeline which draws from the river and pumps it underground 12km to Burra Creek, and then into Googong Dam. It is capable of transferring about 45 Olympic swimming pools of water a day.

But only if there's water to pump. The flow rate of the Murrumbidgee River is now too low to draw from, and the quality of the available water doesn't meet standards. So the pumps must stay off unless the situation improves.

Upstream on the Murrumbidgee river system the drought is biting hard, compounded by an unrelenting draw-down by entitled irrigators and needy country towns.

Dams on the system are falling fast, with Burrinjuck at 33 per cent and Blowering, three times the size of Sydney Harbour, at 55 per cent.

"We made significant infrastructure investments back then and Canberra and Queanbeyan are reaping the benefits of that now," Mr Herzkial said.

"But we're not out of the woods. Our modelling shows that there are adverse conditions ahead. It's all about the need to be vigilant, and take the community on the journey with us."

"The journey" to which he refers is a water-saving one, and Canberrans have an enviable record of responding strongly when required.

After the ACT suffered the double whammy of falling dam levels and the horrendous 2003 bushfires which contaminated supplies, drastic measures were called for.

Restrictions were imposed - a shock to a water-guzzling population previously encouraged to "green" their city - and a range of incentives introduced such as rainwater tank rebates, greywater drain pipe giveaways, and targeted actions such as "gardensmart" and "toiletsmart".

The stated goal was to reduce every Canberran's potable water consumption by 12 per cent within nine years, and by 25 per cent through to 2033.

Still reeling from the 2003 bushfire shock, the city responded strongly. Since then, per capita consumption has fallen by 35-40 per cent, a level that continues even today.

Back in 1997-98, water consumption per head of population peaked at a profligate 215 kilolitres. It has steadily declined since.

"The key message [to the public] is that they've done well but we, both as a company which manages this resource and the public which uses it, can't afford to be complacent," he said.