Prime Minister Scott Morrison has backed a "quick link" airport rail tunnel through Sunshine over the cheaper alternative of existing tracks being considered by the Andrews government.

The federal government has offered $5 billion to build an airport rail link that would see trains travel from Southern Cross to the airport, via Sunshine.

Prime Minister Scott Morrison speaks to the media while Victorian Premier Daniel Andrews looks on . Credit:AAP

During a visit to Melbourne, Mr Morrison confirmed that his government was pushing for a “quick link” to the airport that would include a rail tunnel to Sunshine from Southern Cross.

Rail experts say that a tunnel is crucial to delivering 15-minute city-to-airport services running every 10 minutes, making the train competitive with SkyBus, taxis and ride-share operators.