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.