A major source of money for the Andrews government's $5.5 billion Western Distributor road project could be stoppered by the Coalition and the Greens.

The project, which is currently out to tender, is designed to provide a major road alternative to the West Gate Bridge and ease truck congestion.

But central to the funding of the project is an extension of Transurban's toll concession on CityLink for up to 12 years, which was part of its unsolicited pitch to the Andrews government for the road.

The state is contributing more than $1.6 billion to the project.