The Sniper couldn't be more Australian if you tied him down to a kangaroo and slapped 'im on the barbie.
Also remember that all the TF2 nationalities are parodies. So yes, the Sniper is steriotypically Australian rather than Australian, just as the Spy is a well spoken womaniser Frenchman, the Medic is a German sadist, the Heavy is a big blunt Russian bear, the Engineer is a good ol' boy Texan, the Scout is a Brooklyn scumbag, The Soldier is an American war-hawk, the Demoman is, (aside from being black,) a drunk Scottish brawler and the Pyro is... well we'll see.
Point being, the Sniper is a 100% dinki di Aussie. Australian accent. Australian clothes. Australian language. Australian sensibilities. Australian-themed equipment. Australian background story - not to mention that with the TF2 universe being heavily influenced by Australia, (Saxton Hale, Mann Co, Australium,) it makes perfect sense for there to be a heavily Australian character.