A tunnel between the city and Sunshine could be built as part of the Andrews government's plan for an airport rail link.

Among a suite of massive upgrades to regional rail services, Labor has revealed it is considering building a tunnel between Southern Cross and an upgraded station in Sunshine, as part of the multi-billion dollar airport rail link.

Sunshine station could become a "super hub" that would link to the CBD via a tunnel, according to documents released by the government on Tuesday morning.

"Work is needed to identify options to increase rail capacity between a future Airport Rail Link and Sunshine Super Hub and the city, including a possible new rail tunnel from Sunshine to the CBD," the document states.