South Australia's peak transport industry lobby group has expressed major concerns with a Liberal proposal to build a freight-only airport east of Adelaide to link with new rail and road bypasses around the city.

Opposition Leader Steven Marshall says the new corridor would avoid the heavily populated areas of existing freight routes and is long overdue. He has promised to spend $20 million to investigate full funding options for the project, which would cost billions of dollars, if the Liberals win the 2018 state…