Transurban investors will be "protected" from political bickering when it builds Melbourne's $6.7 billion West Gate Tunnel because the toll road group has signed a compensation deal with the Victorian government, chief executive Scott Charlton says.

The group is raising $1.9 billion in equity to help pay for the project, which includes building tunnels connecting the West Gate Freeway and the Port of Melbourne precinct, a new bridge over the Maribyrnong River, the widening of the West Gate Freeway that connects the west and the city, and the update to the Monash Freeway in Melbourne's South.