A bid by Melbourne Airport's owners to build and run the airport rail line has prompted the former federal consumer watchdog to warn that passengers could be "ripped off".

A consortium including the airport’s owners has made a bid to build and operate the $15 billion project, promising travel times of 20 minutes from the CBD and fares of less than $20, which is being considered by the state government.

Graeme Samuel, who led the Australian Competition and Consumer Commission for eight years, says that a deal with the airport would “entrench a monopoly arrangement” over access to Tullamarine and put consumers at risk of being gouged.

But the airport shot back on Wednesday, saying the rail line would be "fast, frequent, reliable and affordable".

Mr Samuel, who is leading a campaign by airlines for a better deal from the nation’s privatised air hubs, says Melbourne Airport already has total control of access to its terminals by private cars, taxis, Ubers and other ride shares as well as private bus services and even off-site parking providers.