I was interviewed at the Melbourne consulate in February this year, and everyone I saw was in a dark business suit, including a jacket. The guys were in suits with ties, and the girls were in suits with skirts or trousers. If you are really worried about the heat, I would suggest getting there a little earlier so you have a chance to cool down (the Melbourne building at least had a cafe on the ground floor, would expect similar elsewhere), maybe wear a shortsleeved shirt and carry your jacket on the way in. For what it's worth, all the japanese male staff I saw were also wearing jackets that day - on later visits I saw shirts and no jackets. Yeah, wearing a dark suit in Australian summer heat is no fun at all, but I would be surprised to see anyone not doing it, an opinion only based on my experience of course!