A plan to build a multi-billion-dollar underground rail loop connecting Melbourne's western and eastern suburbs via the airport, and link all major train lines, has been unveiled by the Victorian Labor Government.

Key points: New $50-billion train line would link Melbourne's outer suburbs without coming into the city

New $50-billion train line would link Melbourne's outer suburbs without coming into the city The 90km loop would travel from Cheltenham to Werribee via Melbourne Airport

The 90km loop would travel from Cheltenham to Werribee via Melbourne Airport The Government said construction would start in 2022

The new 90-kilometre line, which is being described as the "Ring Road" for public transport, would run between Cheltenham and Werribee via Melbourne Airport and include 12 new stations.

The $50-billion project would allow commuters to travel between suburbs without needing to come into the city.

The Government said it would be "the biggest public transport project in Australian history".

The loop would connect with key stations on every suburban line and feature new regional interchanges for country travellers, and run underground between Broadmeadows and Cheltenham.

Premier Daniel Andrews announced the project via a Facebook video.

The bold plan would link major suburban business and activity centres including Monash, Deakin and La Trobe universities.

Government analysis said the loop through Melbourne's middle ring would also provide more housing opportunities, with the Premier saying some of the cost of the project would be covered by value capture — where local developments benefitting from new infrastructure contributed to the cost.

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It is inspired by major cities around the world such as London and Tokyo that have "polycentric" train networks.

Melbourne's current network is "monocentric" because all routes go to the centre of the city.

In London 21 per cent of weekly travel is made by train, compared to 5 per cent in Melbourne, and the Government hopes the new loop will grow this slice of the pie.

The section of the route between Cheltenham and Box Hill, in Melbourne's south-east and eastern suburbs, will be the first priority.

Both areas are in marginal seats.

Transport Minister Jacinta Allan said initial planning would cost $300 million, with construction to start in 2022.

She said the new line would carry 400,000 passengers a day and take 200,000 cars off major roads.

The Government forecasts the loop would become the busiest train route in Melbourne once completed.

"This will be an investment," Ms Allan said.

"Just as the previous investments in the city loop were made many decades ago, the investments in a ring road were made over many decades ago."

The announcement is the latest in a string of major infrastructure projects from the Andrews Government.

Construction for the $11-billion metro tunnel project, which features five new underground stations in the city, is already underway, while Labor has said work on the much-awaited rail link to the airport could begin by 2022.

Four routes between the city and the airport are being considered. ( ABC News )

Of the four airport rail routes being considered, the State Government favours a line from the city to Tullamarine via Sunshine.

Preliminary works for the $6.7 billion West Gate Tunnel project, which would link the West Gate Freeway to CityLink, have also begun.

Questions over funding, land acquisition

The Opposition said the plan lacked detail, and raised the prospect the Government may have to introduce more taxes to fund it.

"With some estimates suggesting it will cost between $50 billion to $100 billion and a completion date beyond the year 2050, this looks more like a plan for the next election rather than a plan for the next generation," Shadow Treasurer Michael O'Brien said in a statement.

"They have no business case, no engineers' report and they won't rule out more sky rail across Melbourne.

"They refuse to say how many thousands of homes would be compulsorily acquired."

The Opposition said, if it won office, it would refer the project to the independent infrastructure body, Infrastructure Victoria, to see how it stacked up.

A 30-year strategy released by the body in 2016 said train lines in the east and south are not projected to reach capacity by 2046.

"In contrast, lines in the north and west will be under substantial pressure and will require more significant upgrades," the report said.

It called for increased housing density in the south-east, where it saw room to grow.

Infrastructure Victoria CEO Michel Masson said Melbourne's public transport system would struggle to meet the city's needs in future, and new ideas were welcome.

"This proposal is new and was not considered in the 30-year strategy released in 2016," he said.

"So we will be interested to assess it in more detail in light of the long-term needs of the state, and the benefits it brings."

The Opposition's made a series of infrastructure promises of its own. If elected, the Matthew Guy-led Coalitions says it would revive the scrapped East West Link road project, and spend more than $500 million to extend the Frankston and Cranbourne train lines to service one of Melbourne's major growth corridors.

Matthew Guy has promsied to electrify the train line from Frankston to Baxter. ( ABC News )

'Big, bold announcement' from Labor

Mr Andrews said efforts would be made to minimise property acquisitions to make way for the suburban rail loop.

"I won't be the premier when this project is finished, but if we are re-elected I will be the premier that gets this project started," Mr Andrews said.

He said the plan had been developed by Development Victoria with global consultants PriceWaterhouseCoopers, and he hoped there would be a mix of state, federal and private sector investment.

The Government said the project would create 20,000 new jobs during construction.

The Public Transport Users Association said it made sense to focus on the south-east and build the project in stages.

"We've got a bus network at the moment which just doesn't cope," spokesman Daniel Bowen said.

"It'll be up to the Government to explain how they can fund it, stage it and build it."

The new loop would allow passengers to cross suburbs without coming into the city. ( ABC News: Danielle Bonica )

The Property Council of Australia said the proposed suburban rail loop would help Melbourne cope with its booming population.

"This is big, bold announcement by the Government," Victorian executive director Matthew Kandelaar said.

"We've hit five million people in metropolitan Melbourne this week. We're set to hit eight million. This is the sort of project that is really necessary with our growing population."