Half a billion dollars in federal money has been unlocked to turn the Monash Freeway into a 10-lane mega highway.

The national independent infrastructure advisor, Infrastructure Australia, has given the Commonwealth government the green light to invest $500 million for Stage 2 of the Monash Freeway upgrade, warning that continued delays on the road will cost the economy, on average, $180 million a year.

The federal money will make up most of the funding for the $711 million upgrades, with the Victorian government to pay for the rest.

The highway will be expanded from eight to ten lanes between Warrigal Road and Eastlink, and from four to six lanes between Clyde and Cardinia Roads.

Infrastructure Australia approved the business case for the roadworks, which shows the project has a predicted return to taxpayers of $4.60 for every $1 spent.

The federal money for the road was promised in 2016, but the contribution hinged on Infrastructure Australia's assessment of the project's business case.

Infrastructure Australia's chief executive Philip Davies said that without the upgrades, the Monash would face slower and less reliable trip times.

"In our view, now is the time to get on with it," said Mr Davies.

"We've got this increase in traffic, this risk of congestion, which makes people's journeys less reliable which impacts their quality of life.

"It's also impacting on the reliability of the delivery of freight, which costs the economy big time."