Victorians will wait at least five years before construction of a rail line to Melbourne Airport even begins, despite Prime Minister Malcolm Turnbull saying “the time for talk is over”.

But instead, the debate is only just heating up over the preferred route to the airport.

The route that Canberra is leaning towards - to include a massive new rail tunnel from Southern Cross station to Highpoint shopping centre and then the airport - is estimated by state sources to cost more than double the Victorian government’s preferred link.

Mr Turnbull went to Melbourne Airport on Thursday to promise Victoria $5 billion towards the cost of a rail line to Tullamarine, first promised in 1963 but never built.

Melbourne Airport is Australia’s second busiest, with 35 million passengers a year. It is projected to grow to 65 million by 2033.

The Andrews government was dragged reluctantly by Mr Turnbull in 2017 to back an airport rail link, after the Prime Minister said his government would spend $30 million studying the project.

Victoria had expected its preferred route via Sunshine to cost in the order of $10 billion.

But it appears the federal government, which owns a sprawling 127-hectare former Defence Department site in Maribyrnong near Highpoint shopping centre, hopes to include that land in the rail project.

It would mean Highpoint was brought into Melbourne’s rail network. This would require tunnelling of up to 26 kilometres - making that route less likely, as it would cost double the Sunshine route.

Canberra has given Victoria no assurances it will pick up the tab for the unpredictable costs of such a project.

The wrangling over the rail line’s route means it will not start construction before 2023, Victorian Public Transport Minister Jacinta Allan said on Thursday.

Victorian rail and aviation planning experts backed the Sunshine route, saying it would help free up space on other metropolitan rail lines and link to V/Line, connect the city’s south-east to the airport via rail, and get passengers to the airport from the CBD in 15 minutes.

In a statement issued by Mr Turnbull and other ministers on Thursday morning, the “city-shaping project” was described as “our $5 billion congestion-busting investment” that would “help reduce the strain on the Tullamarine Freeway”.

Mr Turnbull wrote to Premier Daniel Andrews early on Thursday morning - after telling selected media about the plan on Wednesday - suggesting a 50-50 funding arrangement with the state.

The plan would use the $2 billion Victoria will get from selling its share of the Snowy Hydro scheme. Mr Turnbull proposed the federal government retain a 50 per cent ownership stake in the airport rail line.

Mr Turnbull’s promise to partly fund the rail line was welcomed by Ms Allan. But she pointed out that the promise, first made in Mr Turnbull’s 2017 budget, needed to be fleshed out.

"It's good the Prime Minister has finally found Victoria on a map, and we’ll accept this funding given how much he short changes our state," she said.

She said there was a lot of work still to be done on a preliminary business case, due to be completed in September.

Opposition leader Bill Shorten said he supported a rail link to Melbourne Airport, but also said careful consideration was needed to select the best route.

“I want to take traffic out of the backyards of that part of north-west melb where I live,” Mr Shorten said. “But I want to make sure we’re not taking people’s backyards as well.”

A preliminary business case is already underway,  and is expected to recommend a route when it is released in September.

The Andrews government's advisor Infrastructure Victoria said in 2016 that the link would be needed within the next 15 to 30 years and would cost up to $5 billion - far less than is now being floated.

The four rail link options the Prime Minister released on Thursday are the same as those discussed in a 2013 report completed by Public Transport Victoria. That report backed the Sunshine route.

The Public Transport Users Association welcomed the airport plan, but said the rail line must be part of the suburban railway network or it would fail. “If it’s not going to suffer the problems of other projects, it’s critical that it's properly integrated,” said president Tony Morton.

Planners also backed the rail line, but warned it needed to be part of a broader transport strategy for Melbourne. RMIT expert Jago Dodson said the funding was welcome news, but warned against pressing ahead with a new airport link that would compete with new roads or disrupt other parts of the rail network.

The RACV’s Bryce Prosser urged the government to ensure the route was fast, cost-effective, and did not put added strain on the rail network.

“To make an airport rail service feasible, it will need to allow for passengers’ luggage and transport passengers between the CBD and the airport within 30 minutes at an affordable price, with frequent services people can rely on," said Mr Prosser, the RACV's ‎general manager of public policy.