House of the Dead: Overkill Extended Cut could be the reason the game was refused classification in Australia. It turns out that's exactly what happened, according to a Classification Board decision report obtained by Joystiq.
The report specifically cites the new Hardcore mode contained in Extended Cut, which requires players to make headshots and evidently ratchets the violence into territory that is "unsuitable for a minor to see or play." The report also mentions Extra Mutants mode which is ... well, it's pretty self-explanatory. Somewhat puzzling, however, is the fact that Extra Mutants mode was also available in the original Wii version of the game.
The Board concludes that "the additional modes included in this modified version and the interactive nature of the game increases the overall impact of the frequent and intense depictions of violence." So there you have it: Classification refused.
Joystiq has yet to receive comment on the matter from Sega, the game's publisher.