Melbourne’s airport rail link will be an “infrastructure white elephant” if it does not deliver 20-minute services via a new tunnel, committees representing Victoria’s biggest cities have warned.

As a decision nears on the final design of the long-awaited project, groups from Melbourne, Ballarat, Gippsland, Shepparton and Wyndham have joined forces to demand a dedicated link running underground from Southern Cross to Sunshine.

They say building the link without a tunnel will leave regional and western suburbs commuters “facing decades of slow and overcrowded trains and continue the crippling dependency on some of Australia’s most congested roads”.

A rail link to Melbourne airport has been long-awaited.It is believed some state government figures want to use existing rail to Sunshine, a cheaper option that would ignore a $5 billion private consortium offer for a tunnel.

It comes as former Labor premier Steve Bracks told the Herald Sun the rail link needed to get commuters to the airport “in a superior time to any other form of travel”.

While he said he did not have a view on the best design, Mr Bracks said it would “not be impossible to have some stops along the way” as long as the train was quicker than driving or SkyBus.

The groups, led by the Committee for Melbourne, said the tunnel was needed to untangle metropolitan and regional rail lines and deliver fast trains to Geelong and Ballarat — expected to house an extra 150,000 people by 2036.

Almost 400,000 people will move into Wyndham and Melton over the same period.