Six tunnel boring machines, each weighing up to 1,000 tonnes, will be used to excavate 2 million cubic metres of soil and rock during the construction of Melbourne's Metro Rail tunnel.

The project involves building a nine-kilometre tunnel and five underground stations, linking the northern and southern sides of the city.

Head of the Metro Rail Authority Evan Tattersall said the tunnel boring machines would work from both ends of the tunnel.

"The tunnel boring machines will be launched from two ends of the job, one around Domain Station ... the other at the Arden end of the project," Mr Tattersall said.

"From our point of view, the project's so important we've got to be working on multiple fronts, and that's why our assessment is six.

"There may be more, there may be less, when the experts get in we'll determine exactly how we'll do it."

He said the machines would construct a concrete shell for the tunnel as they dig out the earth, and would operate around the clock.

"Normally with underground tunnelling operations you work 24 hours a day, and that's why we'll have acoustic sheds over the entries such that we contain the noise," Mr Tattersall said.

Three shafts, each up to 11 storeys deep, are currently being dug in the CBD.

Mr Tattersall said they would be used to lower equipment used to dig out the underground stations.

"The shafts in the city ... will be a different type of construction equipment called a road header that will be able to get in and do the big cavern station construction that we need under the heart of the city," he said.

Construction companies 'desperate' to be involved: Andrews

The Victorian Government also released a shortlist of bidders for the $6 billion public-private partnership to build the tunnel and stations.

The field has narrowed down to three consortia.

Premier Daniel Andrews said there was strong interest in the project.

"These are three of the biggest construction companies in the world who are all desperate to be involved in building Australia's biggest public transport project," he said.

"That gives you a sense of just how significant this undertaking is."

The shortlisted bidders will submit their proposals by the start of next year, and a decision is expected later in 2017.