Victoria wants Canberra to potentially hand over more than $2 billion for the Western Distributor toll road project.
The latest business case for the road extends tolls on CityLink by 10 years, down from a proposed 15, and requires $400 million from Victorian taxpayers.
It also calls for $1.5 billion from the federal government, down from an original $1.8 billion.
But Treasurer Tim Pallas said the national significance of the road could compel Canberra to put in more than just a third of the total cost.
"This is a road of national importance, and it could be that the Commonwealth may want to treat it like other roads of national importance and provide a much more substantial contribution than an allocation in something of the vicinity of a third," he told reporters on Tuesday.
Victoria announced an official partnership with Transurban to build the $5.5 billion Western Distributor, which will connect the West Gate Freeway with CityLink in Melbourne's west.
The road will put an end to trucks driving through inner-west streets to get to the Port of Melbourne, and will impose a curfew on trucks driving along local roads.
The combined tunnel-and-bridge project will cut 20 minutes off commutes each day, as well as take 6000 trucks off the West Gate Bridge.
Opposition roads spokesman Ryan Smith said the project had been announced with "little funding attached to it" and no immediate benefits.
The first stage of the project will begin in December in Melbourne's port, followed by a Monash Freeway upgrade beginning next year.
Construction on the Western Distributor is set to begin early 2018, and the government expects it to be finished by 2022.
