The federal government is moving to widen Commonwealth Avenue Bridge and preparing an "intensive" renewal for Scrivener Dam amid warnings Lake Burley Griffin's infrastructure needs upgrading to avoid costly replacements.
A $14 million pledge in the federal budget would fund the agency overseeing Commonwealth land in Canberra to prepare a business case to widen the bridge leading to Parliament House, and includes $10.6 million to renew the dam on the Molonglo River and remediate walls surrounding the lake.
The funding emerged as part of the Coalition government's package of pre-election budget sweeteners for the city as the ACT's small portion of promised national infrastructure spending attracted criticism.
While the National Capital Authority is looking at widening and strengthening Commonwealth Avenue Bridge, a spokeswoman said it did not expect the ACT government would propose placing its light rail stage two route on the existing thoroughfare over the lake.
The minister for territories, Sussan Ley, on Wednesday said the business case would also consider replacing barriers to make the arterial corridor safe for another 50 years.
Scrivener Dam and the walls surrounding Lake Burley Griffin needed renewal to extend their lives and keep working safely, Ms Ley said.
The National Capital Authority would replace spillway anchors and install new floodgates and mechanical equipment at the dam, which was completed in 1963 to fill the Molonglo River flood plain and create Lake Burley Griffin.
Planning for the "intensive" project to renew Scrivener Dam and the walls surrounding the lake will start immediately, and the authority expects works to commence later in 2019, a spokeswoman said.
The concrete gravity dam and the walls surrounding the landmark water body were functioning safely, she said.
"Budget funding will enable an accelerated program of works that will extend the lives of these assets and ensure they can be maintained efficiently in the future.
"As these assets are now over 50 years old, they inevitably require ongoing repairs and maintenance to extend their useful lives, avoid expensive replacement works and ensure they continue to function safely."
The federal funding, spread over the next three years, will include $3.3 million for the Commonwealth Avenue Bridge's detailed business case, $7.5 million for Scrivener Dam's renewal and $3.1 million towards the remediation of walls surrounding Lake Burley Griffin.
The National Capital Authority expects to finish preparing its detailed business case for the Commonwealth Avenue Bridge in 12 months. It is yet to decide when to begin strengthening and widening the corridor and replacing its safety barriers.
Commonwealth Avenue Bridge was an obstacle in talks between the ACT government and the authority as planning for light rail stage two proposed extending the burgeoning network from Canberra's city centre to Woden.
After receiving a territory government plan to take over a traffic lane on Commonwealth Avenue for light rail, the authority was concerned about the impact on the wider city road network as well as the ceremonial routes. The ACT government has since changed its model to include building a new, separate Commonwealth Avenue bridge and use the median strip of the existing structure.