"We've run afoul of certain prohibitions regarding the depiction of drug use," writes Undead Labs' founder Jeff Strain on the game's forum. "We're working with [publisher] Microsoft to come up with options, including changing names of certain medications in the game to comply with ratings requirements. Whatever our path forward, it's going to take a bit."
Coincidentally, the apocalyptic world of Fallout 3 ran into the same drug-naming issue with the Australians back in 2008. After some editing, the game was suitable for distribution.
"Microsoft is currently evaluating the options with regards to the title's classification," a spokesperson for the publisher told Joystiq. "The Australian Classification system plays an important role in ensuring that Australians can only access age-appropriate games and content."
Strain acknowledged as much in his post, saying, "I know this is frustrating – believe me, we're frustrated too – but each country has the right to set its own rules about content, and it's our responsibility to comply with them. Rest assured we'll do everything we can to find a way to get the game into your hands."