The Walking Dead would have been published in Australia even if the new R18+ mature rating didn't exist, as the Classification Board has given the game an M15+ rating.
While the game was available on Steam and iOS before it was rated, the game was not released on Xbox Live Arcade and PlayStation Network yet; now that it has been given a rating, it'll be available for purchase in the region.
Oddly, Telltale Games assumed The Walking Dead would be too violent to get an M15+ rating, and delayed getting the game rated until the R18+ classification was ratified. Back in April 2012, Telltale stated in its forums, "Sorry, but due to the OFLC ratings laws in Australia and New Zealand, and the fact that this is a mature game, we do not currently have plans to release the game there on consoles."
The irony is bittersweet.