The Andrews government has been warned it will need to invest billions on a new rail link between the airport and the city in future years, or hope the Tullamarine Freeway can continue to carry the load, despite warnings from the airport that it faces gridlock within a decade if a rail line is not built.

Melbourne Airport believes a rail link is needed within 10 to 15 years.

The government's $10.9 billion Melbourne Metro tunnel was designed to create capacity for Melbourne's rail system to support a link to Tullamarine, with airport trains proposed to run via the planned Metro tunnel some time after it opens in 2026.

But the tunnel must also handle booming demand on the Sunbury and Melton lines, both of which serve rapidly growing populations.

Melton line passenger numbers are expected to quadruple within 15 years, overtaking the busy Frankston line, the Melbourne Metro Rail Authority says, and numbers on the Sunbury line have grown at a runaway rate of 12 per cent a year.

Both those lines will feed into the Melbourne Metro tunnel.

This soaring demand has forced state rail planners to rewrite long-term service plans for the tunnel, and an airport line has effectively been jettisoned.

As recently as 2013, Public Transport Victoria published plans for the Metro tunnel that included an airport link with six trains an hour.

But the most recent service plan for Melbourne Metro, published in the 2016 business case, makes no mention of a rail line to Melbourne Airport.

Rather, it predicts the tunnel will be used by up to 14 trains an hour to and from Sunbury and nine trains to and from Melton. It will handle 23 trains an hour, which would leave no capacity for an airport line.

The government now faces calls to build an entirely new tunnel for airport trains.

The proposal, by the Rail Futures Institute, involves building a new line between Southern Cross Station and Sunshine, mostly underground. The new line would also have new dedicated trains with luggage storage.

Put to the Public Transport Minister Jacinta Allan in recent weeks, the airport link would be a major investment for the Andrews government, which has never committed to building a rail line to Melbourne Airport.

This is despite predictions from the airport that the Tullamarine Freeway will soon struggle to handle peak-hour traffic, despite the current $1.8 billion widening project.

Melbourne Airport predicts it will have 64 million visitors a year by the early 2030s and says the freeway will fail to cope with peak demand by then.

A spokesman for Melbourne Airport said it supported building an airport rail link.

"We need a detailed study into feasible solutions now because construction will take a decade and realistically the solution will need to be operational in the next 10 to 15 years," he said.

Rail Futures Institute secretary Bill Russell, a former adviser to the Bracks and Brumby governments, agreed, but said it was increasingly clear Melbourne Metro was not the answer.

"The growth in usage from those two lines [Sunbury and Melton] will absorb the capacity for Melbourne Metro, so there really is a need for further capacity to serve the airport," Professor Russell said.

"Otherwise airport trains will just be jacked up behind suburban trains going at walking pace into the city."

The Department of Economic Development, Jobs, Transport and Resources declined to say how many trains an hour would use the Metro tunnel.

"The project will create space to run more services to Sunbury, Melton and any future airport rail line," spokesman Lewis Hill said.

"Specifically how many services will be determined closer the completion of the Metro Tunnel in 2026, and any airport rail project, which Infrastructure Victoria recommends be built somewhere between 2030 and 2045."