According to this Gamespot report, the only differences anyone's found between the Australian and European version's of GTA IV so far is in the way certain aspects of prostitution are dealt with in the game. From the site:
Having just received a region one version of the game, we tested out a few scenarios and only found one difference so far: how the game deals with prostitution. In Australian versions of GTAIV, Niko can indeed pick up prostitutes, but once he takes said sex worker to a secluded area, the game camera shifts to a tight shot of the rear of the vehicle the pair are in and cannot be moved.
In the region one version, once Niko finds a secluded spot, an extra option pops up, asking whether Niko wants the $20 or the $50 service. Once the appropriate "service" is chosen, the act begins, but instead of a static camera, players can freely rotate the game camera around the vehicle.
Neither Rockstar or the OFLC have released details of cuts made to the Australian version as yet.