Builders started clearing a site at Footscray on Wednesday where boring machines will begin digging the tunnel from early 2019.

The construction effort will be largely funded by toll road giant Transurban, which approached the state government with the idea.

Neither the tunnel plans or a concession deed to extend Transurban's tolling of CityLink to help fund it has passed parliament.

But that doesn't worry Premier Daniel Andrews, who visited the construction site on Wednesday.

He says there is no need for legislation to pass until the road is finished in 2022.

And if the CityLink tolling plan doesn't get a parliamentary tick of approval, the state will pay for the tunnel itself.

"In the event that there is not sufficient numbers in the parliament, then instead of users of the road paying for it, every Victorian taxpayer will. It's as simple as that," Mr Andrews told reporters.

The opposition labelled the move "bullying", with leader Matthew Guy citing a Liberal plan to move tunnel-related motions when parliament resumes in February.

"For the government to try and proceed with a major road project without any legislation, would have to be one of the first times I've seen that used in Australia," Mr Guy said.

But Mr Andrews said the start of works was about finally delivering a project that has been talked about for too long.

"Unless you build this alternative, you will never get trucks out of local roads," he said.

About 350 homes will be above the path of the 18-metre deep machines that will dig and construct the tunnel, moving at about nine metres a day, around the clock, from early 2019.

Roads Minister Luke Donnellan said there will be assessments of homes before and after construction starts, but builders aren't expecting the machines to cause any damage.