More voices are joining the chorus for a rail tunnel from Melbourne Airport between Sunshine and Southern Cross Station, to unlock opportunities for regional Victoria.

The Committee for Ballarat is part of a coalition of development committees and councils pushing for the tunnel as part of the state government's airport rail plan, and on Monday, the group announced the Committee for Melbourne and Committee for Wyndham were in full agreement.

The City of Greater Geelong released the Stronger Together report in June, supported by councils in Ballarat, Bendigo, Latrobe Valley and Shepparton as well as the development groups, calling for fast rail and the airport tunnel.

Committee for Ballarat chief executive Michael Poulton said it was good to see metropolitan bodies join the coalition.

"It's significant," he said.

"We've always known it's beyond regional Victoria, it's about opening up opportunities for Melbourne to get to the regions as well."

Building the tunnel from Sunshine, and including access for regional services, would join other initiatives like the Ballarat Line Upgrade and the state government's Western Rail Plan to fully separate metropolitan and regional trains.

Other infrastructure, including electrifying and quadruplicating tracks to Melton, would also be required, as well as new rolling stock.

However, Mr Poulton said any attempt to build fast rail to Ballarat or Geelong would be stymied without the tunnel out Melbourne - metropolitan trains stopping at each station would slow down regional trains.

Without the regional trains, there's space for more metropolitan services, he added, which is good news for the booming inner-west suburbs.

In a media release, the coalition said the "wrong solution will leave the growing 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".