Copied the following from PTV site.
Free travel on Christmas and New Year’s Eve
Added: 12 December 2018
Public transport will be the best way to enjoy the festive period with free travel on Christmas Day and New Year’s Eve across Victoria.
Free travel on our trains, trams and buses will make it easier, safer and cheaper for you to celebrate with family and friends.
From 3am Christmas Day until 3am Boxing Day you will be able to travel on any metropolitan or V/Line service without paying a fare, with services running to a Sunday timetable.
Public Transport, including regional town buses, will also be free from 6pm New Year’s Eve until 6am on New Year’s Day.
After midnight, trains will run every 10 minutes until 1.30am with extra services running through the night. Until 2am, trams will run every 5-10 minutes with buses also departing the city every 15 minutes.
If you're in regional Victoria, V/Line will operate additional services as well as night coaches from Southern Cross. There is also free travel on the first V/Line service to each regional destination on New Year’s Day.
You won't need to travel with a valid myki or touch on during free travel periods. If you do happen to accidentally touch on, you will not be charged.
Further detailed service information will be provided in the coming days.
in Adelaide, public transport is free when there's a complete shutdown of train services.
it's free because you can't use it therefore spend no money and have to walk.