If you buy an AU machine it'll play AU or EU games which are in the PAL format. US and JP games are in NTSC and won't play.

As Tetsuwan said, there are means of hacking the system to play all regions though.
However, i'd do some research first on (gbatemp forums), since newer models might be less hackable than older systems. It's tough to keep up since the 'scene' moves so quick!