Found out on 6pm news.
Do they offer this free transport every year? Not sure.
I think it's a great incentive to leave a car at home.

Free travel across the network

You can travel free on Victoria’s public transport network from first to last service on Christmas Day (25 December 2012) and after 6pm on New Year’s Eve (31 December 2012) until the first normal

Great 2 day offer: iPhone and iPad Sydney public transport planning for only 99c

The Victorian Government is pleased to provide Victorian Seniors Card holders with free public transport during the 2012 Victorian Seniors Festival.
Eight days for metropolitan/regional (7-14 OCT)

The first 36 eligible posts in this thread asking for a promo code will be sent one via OzBargain. Yeah just 36 this time, sorry :(

There are just a few conditions:

Passengers can travel free on Victoria’s public transport network on Christmas Day (25 December) and New Year’s Eve (31 December 2010).

Free Myki Card for the new Victorian public transport system which will replace Metcard. At the end of the promotional period these will be $10 (or $7 concession).

A 5% discount if you purchase through metlink online.
Start from 26 July 2009 - 08 August 2009.
discount is not a lot, but it's better than nothing.

Just letting people know that anyone attending the NAB Cup Bushfire Appeal Match on Friday 13th Feb will be able to use free public transport. You must have a ticket for the event.

Present this voucher onboard and get all day light rail pass for $6

Melbourne's public transport network will be FREE from 6pm on New Year's Eve until the first normal scheduled service the next day.

This means : TRAINS - TRAMS - BUSES and V-LINE

Free public transport on Melbourne's trains, trams and buses. Christmas Eve and Christmas Day only! Enjoy.

In the paper,
http://www.news.com.au/heraldsun/story/0,21985,24836719-661,...