Mr Andrews on Tuesday revealed a massive and unexpected blowout in the cost of the West Gate Tunnel, with projected costs escalating to $6.7 billion, up from last year's estimated $5.5 billion.

With Labor moving to stop the road becoming an issue at next year's state election, Transurban locked in a major victory for its shareholders: the guarantee of a big government payment if funding is blocked by the Parliament.

The Premier announced on Tuesday the government had signed contracts, with work starting in January.

The project will widen the West Gate Freeway to 12 lanes, build a new tunnel under Yarraville, get 9000 trucks a day off local roads in the inner west, and give motorists more routes into the city centre.

To pay for it, the government wants Parliament to approve a deal to extend Transurban's generous 4.5 per cent annual increase in tolls for 10 more years, starting in 2019.

Tolls rises would otherwise have fallen into line with inflation, less than 2 per cent in the past year.

And Transurban will get an extra decade of charging motorists to use the road if Parliament approves the deal. Tolling on CityLink finishes in 2035 under current legislation; this would be extended to 2045.

Victorians will pay $2.685 billion of the road's construction costs, while Transurban will cover $4 billion. But in return Transurban will get an extra $15 billion in revenue, the opposition estimated.

To extend the contract, the CityLink Act must be changed in Parliament.

The state opposition and the Greens have vowed to overturn in Parliament's upper house any changes to the act passed in the lower house.

Opposition Leader Matthew Guy labelled the agreement with Transurban "one of the worst contractual deals signed by a government in living memory".

Mr Andrews said Victorian taxpayers would foot the entire bill for the project if the Liberals and Greens followed through on threats to block the new tolling deal.

"It's very simple," Mr Andrews said. "Either this road will be paid for by motorists or it will be paid for by every single Victorian taxpayer. Either way, work starts in the next couple of weeks."

Transurban said in a statement that, if the Victorian Parliament did not extend CityLink tolling for a decade, it would get "completion" and "substitution" payments from the government.

These payments would cover "construction costs, financing costs and a return component commensurate with Transurban's investment metrics and the risk profile", the statement said.

This funding guarantee for Transurban follows a controversial "side letter" signed by the Napthine government in 2014. The side letter guaranteed the consortium delivering the East West Link project a payout if the road did not go ahead.

Mr Andrews rejected comparisons to the East West Link. The West Gate Tunnel will be built regardless of any parliamentary vote.

The construction companies building the road will get about $5.3 billion to deliver the works. The remaining $1.4 billion will be spent on project management, property acquisition, legal fees and contingencies.

The project also includes a new bridge over the Maribyrnong River and an elevated road above Footscray Road.

Transurban's statement to the stockmarket confirmed that parliamentary approval was needed to secure an extra decade's tolling of CityLink to help pay for the project.

"Separate West Gate Tunnel Project legislation is required to align the project toll enforcement regime with CityLink [tolls]", it said.