The Melbourne Metro rail tunnel will mostly be built as a public-private partnership, with the state government on the hunt for investors willing to finance much of the $9 billion-$11 billion project up-front.

In exchange, they will get service payments from the state government for operating and maintaining the nine-kilometre rail tunnel once it opens in 11 years.

Availability payments have recently been used in Victoria to build Peninsula Link, a 27-kilometre freeway south-east of Melbourne, the $5.7 billion desalination plant, and Southern Cross Station.

The model helps governments avoid going into debt to fund major projects, but can increase the long-term costs significantly, in part because private-sector finance attracts higher interest rates than state debt.