Work will primarily involve strengthening the dissipator structure, which is responsible for absorbing the energy of water flowing over the flood gates.

The complex undertaking will mean work is carried out on the spillway - the part the water spills from - while maintaining the function of the dam.

The lake will potentially be lowered as a result, while the work is done.

National Capital Authority lakes and dam manager David Wright said reducing the flow of water would be flood management to mitigate against an extreme weather event.

"To keep it in context, we will be lowering the lake less than the levels that we saw in late 2019, which was due to the drought," he said. "We've got the lake users front of mind there, to make sure we don't impact any communities too much."

Mr Wright said studies, which examined the impact of relatively minor floods to once-in-560,000-year floods, were undertaken to determine what upgrades were required.

"Under certain hydraulic conditions, certain flows, that stability of the downstream structure might come into question," he said. "It's not a safety risk, but it is something that needs to be addressed."

The NCA has owned and managed the dam on behalf of the Commonwealth government since it was built in the 1960s, in conjunction with a series of lake walls and bridges specifically designed to create Lake Burley Griffin.

While Scrivener Dam has had regular maintenance, and 120 anchor bolts were replaced when they were discovered as corrosive in 2011, the structure has not undergone significant work since it was built.

Under the funding model for major NCA assets, periodic larger investments are required, in addition to annual maintenance funding, to reduce the risk of asset failure and to extend the life of structures in the most cost-effective way.

The work was announced in the 2022-23 Budget package for the ACT, at a cost of $38.5 million.

Mr Wright said while the dam doesn't play a major role in the health of the lake, La Nina had been a double-edged sword.

He said high rainfall meant a higher influx of nutrients flowing into the lake, which could increase algae blooms down the track.

"This year we've had a nice, cool summer which has meant that algae can't really bloom as it usually would," he said.

Mr Wright said, given the changing climate and increased extreme weather, planning for conditions considered very rare was required.

He said the detailed design and heritage and environment assessments could take up to 10 months.

Construction would likely begin in 2023.

Mr Wright said it would likely take 10 to 12 months for work to be completed.

"What makes this project quite complex is that we do need to maintain the function of the dam, we've still got a river, it is still flowing," he said.

"We want to minimise disruption, as much as we can, to the lake and to the lake users."

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