The tempo has lifted at Scrivener Dam after a government funding injection

In a corridor deep inside, there's an abrupt, sharp thumping as motorists drive above.

Stand outside at Scrivener Dam in the dead of night and a lion breaks the silence with a roar from neighbouring Canberra Zoo.

The buzz of machines and tools has also filled the air as contractors have sandblasted and stripped old paint from a floodgate in need of refurbishing.

An injection of federal government money in 2019 lifted the tempo of work to maintain the dam, now 56 years old but no less responsible for giving form to the lake.

The National Capital Authority's director of estate management, Peter Beutel, says while the arrival of new electrical controls more than 18 months ago made for a big "infrastructure year" at Scrivener Dam, the pace has been fast in recent months.

"We're doing a lot more little jobs. A lot more jobs that don't cost a million bucks," he says.

For a dam that creates a lake reaching 40km in shoreline across the centre of Canberra, there is a rolling agenda of maintenance that changes for its operators each year.

"You have this big long list of jobs you want to do, and you have a budget you can do it with, and you have to prioritise, and that prioritisation changes, and so does your budget," Mr Beutel says.

The $7.5 million windfall from the government last May has let the NCA get ahead on that to-do list, doing jobs that might have waited.

It would typically work on between six to nine dam maintenance projects per year. With the funding, that grew to 19 last year.

"Just at the moment we're getting a lot of things done," Mr Beutel says.

On the agenda is more security to stop fishers from using a nearby jetty where operators usually store a 70-tonne, floating bulkhead that plugs the dam when one of its floodgates is lowered for maintenance.

The bulkhead is now fitted into the dam while contractors sandblast, rebuild seals and repaint one of its five floodgates, the last in line to undergo a round of maintenance.

A sluice gate at the bottom, one of three controlling the lake's water level most of the time, is being refurbished too. It's the kind of project brought forward by the additional government funding last year.

For all the din of the maintenance work, the dam gets noisier in flood. Its operators open its floodgates, at the top of the dam, when water inflow is too high for the sluice gates to manage alone. Spectators gathered to watch water pour into the Molonglo River and speed away when the floodgates opened in 2010.

All five floodgates have been open at the same time only once in Canberra's history, when floods hit the city in 1976. Even then, they were only partially open.

A set of concrete blocks protruding like teeth at the dam's base sit waiting to soak up the force of the rushing water before it streams, resuming the form of a river, through the Molonglo Valley.

There seemed little chance of that spectacle earlier in 2020, when drought had shrunk Lake Burley Griffin's height.

Water levels move up and down for irrigation dams. Scrivener Dam is made mainly for other purposes.

It's designed for precision, keeping the lake at 555.93 metres above sea level, give or take 100mm. Apart from water used to irrigate Commonwealth Park, the Royal Canberra golf course, and the Australian National Botanic Gardens, dam operators let the rest out.

"What comes into the dam, goes out of the dam, to keep the lake level at a constant height," Mr Beutel says.

"So for all intents and purposes we run our lake like a river."

Earlier this summer drought slowed its inflows, which come from the Molonglo River, and further along, the Queanbeyan River, Googong Dam and Captains Flat. Other tributaries, including Jerrabomberra Creek and Sullivans Creek, were also running emptier in December and January.

The lake was 510mm below its normal level earlier this year. After the hailstorm on January 20, the figure changed to 485mm. Last Wednesday, ahead of the weekend's rain, it was 550mm below normal lake level.

The natural shallowness of Lake Burley Griffin, created on a floodplain rather than ravine country, raises the stakes for Scrivener Dam.

Its floodgates, 30m by 5m in size with bellies rounded like a bloated guppy, hold up the top five metres of lake water.

If one failed and let water drain, East Basin, Central Basin, West Basin and half of West Lake would become mudflats. Only Yarramundi Reach and Tarcoola Reach, at the western end of Lake Burley Griffin near the dam, would be deep enough to have permanent water.

Such nightmare scenarios are what Mr Beutel and National Capital Authority manager of lake and dam, David Wright, plan against.

"Because our floodgates are charged all the time, they have water against them all the time, we have to maintain them to a high degree," Mr Beutel says.

More than 20 hydrometric stations up and down the catchment monitor rainfall and water discharge at those points, letting flood modellers predict how much water the dam should let go to prevent flooding.

It's more than 18 months since the dam completed what Mr Beutel regards as a legacy project, replacing its electronics controls with a new system.

Before, the technology was decades old.

"You could press the button, and the gate came down, and you pressed another button, and the gate stopped coming down. So, this far short of cranking the handle," Mr Beutel says with a smile.

The new controls aren't visually spectacular, but they let dam operators use their phones to see how much water's running through on video, and monitor lake level calculations.

However, there is no access to the computer system from outside.

An electronic interface inside the dam monitors water levels, and lets operators move the gates and change the sluices.

"They can run it automatically, they can run it manually, they can isolate and turn it off," Mr Beutel says.

The sluices function like taps, letting water through.

"A computer drives that, and the computer picks up the lake height and says, 'right we need to let water out or we need to stay static'. And that basically operates the dam 95 per cent of the time."

The electrical upgrade would make both Canberrans and people working at the dam safer, he says.

"Not photo worthy, but it's one of those million dollar projects to basically bring the dam into the 21st century."

Mr Beutel insists on 24 hour operations in flood situations, so staff are on hand to inspect floodgates and see they're fit to be lowered.

In case of a power failure, the dam has a back-up generator. And in case the back-up generator fails, operators can truck in a genset, an electrical generator and engine combined and mounted together.

"And if the genset fails, we can get another one, but we could physically wind up through a portable unit, wind the gates back up and close the gates back up if we had to," Mr Beutel says.

"So we have back-up, upon back-up, upon back-up, as you would expect on a major piece of infrastructure."

While precision is important for a dam holding Lake Burley Griffin at a stable level, safety is more important, Mr Beutel says.

The dam operators want to create the right environment for the boaters, kayakers and others using the lake, he says.

"That's important to us. So we can facilitate that kind of activity.

"The rowers, when they launch their boat, if they don't have a floating pontoon that moves up and down with the lake, they launch from concrete walls.

"That concrete wall becomes a little bit less easy to use if the lake level is going up and down all the time."