That's not the case in all Australian buildings. Usually we call the ground floor "ground floor" (which you could argue is 0-indexed), and the floor above it is

That's not the case in all Australian buildings. Usually we call the ground floor "ground floor" (which you could argue is 0-indexed), and the floor above it is "the first floor". But there are a lot of buildings that follow the American style of "the ground floor is the first floor". We also don't use negative numbers, basements are prefixed with a B, so B1 is like -1.