In case you're unfamiliar, there is no adult rating for video games in Australia -- with the maximum being MA15+. Any mature title that is unable to obtain the Classification Board's approval for MA15+ is barred from commercial sale. Oddly, the original Left 4 Dead made it to shelves with the Board's blessings.
This doesn't necessarily mean Left 4 Dead 2 won't eventually be given the OK, though. I assume if Valve edits out the content in question, the ban will be lifted. Other titles to be forbidden in recent times include "Sexy Poker," "Necrovision," and "Risen."
You can read the Classification Board's report here (PDF) and its website can be found here. Don't hesitate to leave your rants or otherwise in the comments.