Yeah, it's kinda funny that the trend to become informal in Australia since I came in 2004 (when a few consultants still expected white coats) has been driven by an interest in infection control. Or at least, that's been the excuse cited. Ties went out last decade, at first among paediatricians (whose patients grabbed and slobbered all over them). Now many hospitals PREFER short sleeve shirts because cuffs spread germs and get in the way of vigilant hand-washing. And after a number of years of contradicting the infection control imperative, finally, many have switched to disposable tourniquets.
In rural hospitals at least, you'll come across many docs wearing t-shirts and/or long shorts, but I wouldn't advise you try these as a student. Whether you can get away with a short sleeved shirt will depend on your hospital and consultants. I'd recommend bringing a few, but of the collared, button-down variety.