Melbourne motorists will pay tolls for an extra 10 years on CityLink to help pay for the West Gate Tunnel Project, which has blown out to $6.7 billion.

The Victorian Government has signed the contract for the road linking the West Gate Freeway to CityLink, despite the State Opposition's threat to block the tolling deal with infrastructure giant Transurban in the Parliament.

Transurban will contribute $4 billion to the project, in exchange for motorists paying tolls on CityLink until 2045, under a deal the Opposition estimates will rake in $15 billion in revenue.

The toll road was initially estimated to cost $5.5 billion, but will cost $1.2 billion more because the twin tunnels under Yarraville are now twice as long as originally planned.

Premier Daniel Andrews said taxpayers would foot the bill for the whole project if the Liberals and the Greens carried out their threat to block a 10-year extension to Transurban's existing CityLink concession deed.

"This road will be paid for by motorists or it will be paid for by every single Victorian taxpayer," he said.

"Either way, work starts in a couple of weeks' time."

'Worst deal in living memory': Opposition

The state will contribute $2.6 billion to the second river crossing, but if the funding arrangement does not pass the Parliament, Transurban will receive taxpayer-funded "completion" and "substitution" payments.

West Gate Tunnel Project: New tunnels from West Gate Freeway under Yarraville

New tunnels from West Gate Freeway under Yarraville Widening of West Gate Freeway from eight to 12 lanes

Widening of West Gate Freeway from eight to 12 lanes 14km of new and upgraded cycling and walking paths

14km of new and upgraded cycling and walking paths New bridge over Maribyrnong River

New bridge over Maribyrnong River New elevated road along Footscray Rd

New elevated road along Footscray Rd Better access to ports, removing trucks from residential areas

Better access to ports, removing trucks from residential areas To be completed by 2022

"Completion/substitution payments cover project construction costs, financing costs and a return component commensurate with Transurban's investment metrics and the risk profile," the company said in a statement to the stock market.

Opposition leader Matthew Guy said the agreement was "possibly one of the worst contractual deals signed by a government in living memory".

"Daniel Andrews is Transurban's golden goose," he said.

Greens leader Samantha Ratnam said the Andrews Government had made an extraordinary decision to proceed with the project, generating huge profits for Transurban.

"It goes against everything they said they stood for when they went to the last election concerning the East West Link," she said.

"We're going to do everything that we can to stop this tunnel, this waste of money, when we have better solutions for congestion."

Mr Andrews rejected suggestions the project had parallels with the Napthine Government's East West Link, which was dumped by Labor at a cost to taxpayers of more than $1 billion.

"The key difference is this project stacks up and the other one didn't, and we're not signing contracts five minutes before the election," he said.

"This project returns a dividend on every dollar we invest."

CityLink tolls will rise by an annual 4.25 per cent from 2019 to 2029 and then in line with inflation, with similar fees on the West Gate Tunnel.

Motorists driving into the city from the tunnel will be tolled at Footscray or Dynon Roads between 7:00am and 9:00am.

Transurban's chief executive Scott Charlton would not answer questions about the project today.

The company said the road would take 28,000 vehicles off the West Gate Bridge and 22,000 off the Bolte Bridge, and more than 9,000 trucks off local streets every day.

The project is due to be completed in 2022.