Blacklight: Tango Down, a recently-released online shooter, hasn't been released in Australia. Why? Because it's an online shooter.
It's a strange situation for a game to find itself in, but that's definitely the reason Blacklight isn't available for Australian consumers; because the country's Classification Board will not rate a game's online component, there was nothing left for it to rate, and an unrated game is not allowed to be sold in Australia.
In some ways, it's similar to the ESRB's "experience may change during online play" caveat, only a lot more damning. Despite this being Australia, there's no connection between the lockout and the game's violent content; it's simply down to the fact that every single game mode, even the singleplayer, requires an Xbox Live connection.
So why hasn't this happened for other games? Most titles have at least one offline component, whether it be a tutorial or skirmish mode. Older online-only games, like World of Warcraft, got by under old guidelines that stated they didn't have to be rated as video games (for a good explanation of why, see here).
That loophole was closed six months ago, however, and Blacklight has been the first game to run foul of them.
The good news in this case is that Microsoft and developers Ignition have supposedly found a workaround, and the game is being submitted to the Classification Board very soon. So hopefully for Australian and New Zealand customers, you'll be able to try the game in a month or two.
The bad news? The Australian Classification Board's capped ceiling of an MA15+ rating is, while ridiculous, at least explainable for the fact it was an innocent oversight implemented in the days of Pong and Pitfall (which supposed games would never need an adults-only rating). This change, though, is recent, and somebody, somewhere should have known better, especially as more and more games go online-only.
[<a href="Blacklight Tango Down FAQ">Xbox Australia]