Independent transport experts advocating for a stand-alone rail link to Melbourne Airport via Sunshine say planning for the project must begin immediately, despite the Andrews government maintaining it won’t be needed for another 15 to 30 years.

On Friday, the state government welcomed Prime Minister Malcolm Turnbull’s new interest in building an airport rail link following reports that next month’s federal budget could include money to kickstart the long-awaited project.

But Victorian Public Transport Minister Jacinta Allan said the $10.9 billion Melbourne Metro tunnel needed to be completed first.

The tunnel has been designed to free up capacity on Melbourne’s rail network and eventually support a link to Tullamarine that would branch out from the Sunbury and Melton corridor at Albion. It’s due for completion in 2026.

But a proposal by the Rail Futures Institute (RFI) says Melbourne Metro was not a prerequisite for a rail line to the airport.

The institute’s Bill Russell, a former advisor to the Bracks and Brumby governments, said a new line between Southern Cross and Sunshine, built mostly underground, was needed because the Sunbury and Ballarat lines could not absorb the extra capacity required to serve the airport.

“There needs to be a stand-alone line and planning needs to start now,” he told Star Weekly.

“Sky Bus is regularly taking 45 minutes for the journey [from the CBD to the airport] – it’s not really competitive with the 15 minutes the rail link would take.”

In its submission to Infrastructure Victoria’s (IV) recent 30-year blueprint, FRI calls for a high-speed train on a segregated line from Southern Cross to Tullamarine that would provide reliable 15-minute journeys at 10-minute frequencies, stopping only at Sunshine for interchange with Melbourne Metro and V/Line services.

In IV’s strategy, a rail connection to the airport was listed as a mid to long-term project. The Andrews government has not made a firm commitment to build the rail link.

Melbourne Airport would not comment on RFI’s proposal but a spokesman said a rail link from the airport to the city was needed within 10 to 15 years.

“We need a detailed study into feasible rail options now, because construction will take a decade and realistically the solution will need to be operational in the next 10 to 15 years.”