The federal government will tip in $3.25 billion towards Victoria's $26.1 billion North East Link toll road.
Prime Minister Anthony Albanese confirmed the money for the blown-out project alongside federal Infrastructure Minister Catherine King and Premier Jacinta Allan on Thursday.
"Next week's budget will contribute an additional $3.25 billion for this project, bringing the Commonwealth investment up to $5 billion," Mr Albanese told reporters at Watsonia.
"My government will work with state and territory governments across the board on nation-building infrastructure, such as this project here, which is really fitting in a missing link in the Victorian road network."
The funding will go towards the non-tolled parts of the road, including the M80 Ring Road connections and Eastern Freeway upgrades.
The major road project in Melbourne's east was originally tipped to cost $10 billion when it was unveiled in 2016 and priced at $15.8 billion in the 2017 business case.
In December, a new price tag of $26.1 billion was revealed.
Construction is under way on the 10km-long connection between the Western Ring Road and an upgraded Eastern Freeway, which is set to be as wide as 20 lanes in some sections and includes 6.5km of tunnels.
"The North East Link project has been talked about for such a long time, it's been talked about for decades," Ms Allan said.
"Indeed, it's been a dotted line on the Melway map for longer than I've been alive."
The state government signed a $11.1 billion deal in 2021 with Spark consortium to build, operate and maintain the project's tunnel toll road.
It is expected to cut travel times between Melbourne's north and southeast by up to 35 minutes and take 15,000 trucks off local roads each day when completed in 2028.
The announcement comes ahead of the federal budget on May 14 and after the state budget was revealed on Tuesday.
In his 10th budget, Victorian Treasurer Tim Pallas revealed Melbourne Airport Rail will be completed at least four years later than its original 2029 target.
Construction on the mega project has stalled as the government and airport remain at odds over whether the Tullamarine station should be underground or elevated.
On Wednesday Mr Pallas suggested that if the the stand-off continued, the state might start shifting its efforts to making Avalon Airport towards Geelong more accessible.
Mr Albanese on Thursday said the federal government would not be intervening.
"We don't control Melbourne Airport," he said.
"It is Commonwealth land, which is leased. That privatisation decision was made some time ago."
Melbourne Airport had previously been part of a consortium that offered up to $7 billion towards an underground station but it was rejected by the government.
Australian Associated Press
