Myki will become the only ticket valid for Melbourne's public transport system on December 29.

Metcard validating machines on trams will be switched off the day before, and Metcards will cease to be sold at train stations by the end of October.

Expiring soon ... Metcard.

More than 85 per cent of public transport users already use Myki, up from 20 per cent at the start of the year.

Transport Ticketing Authority chief Bernie Carolan said the biggest remaining challenge lay in convincing the one-in-four bus passengers that still use Metcard to switch.