The long-demanded rail link to Melbourne airport will be included in an expanded Metro rail tunnel plan that Premier Denis Napthine has promised will begin this decade.

Speaking at a Property Council lunch in Melbourne on Wednesday, Dr Napthine said: ''Let me absolutely assure you, our government is committed to a rail capacity project, to enhance rail capacity through the centre of Melbourne, to boost rail capacity on the Dandenong line and the Gippsland line. We need it, it is essential.

''We are committed by the end of this decade to be building a rail capacity project and include in that a link to the airport.

''We will deliver that because it is absolutely essential, so don't let anybody have a misunderstanding about our commitment to that as a key project.''