Construction will begin on a rail link between Southern Cross Station and Tullamarine airport within less than a decade, the Andrews government says.

The Melbourne Airport rail link will also bring huge benefits to country Victoria, Premier Daniel Andrews will announce on Thursday, by untangling the V/Line rail network from congested metropolitan tracks in Melbourne's west.

Planning is set to begin on building an airport connection that will also free up trains on the Geelong and Ballarat lines, and reduce travel times between the two regional cities and Melbourne.

Journey times between Melbourne and Geelong could be cut to less than 40 minutes, under a model for airport rail being considered by the Andrews and Turnbull governments.