I often wondered the same myself.
Australia is not upside down. A little racist, bigoted and intolerant at times, but never upside down. Unless, it is 4am on a Saturday morning when all the pubs have had to close, and the drunks fall out into the gutters. Then things might be upside down.
If you look on a map, Australia is pointing the same way as all the other continents. The writing on the map is the same way too, thus proving my case (well, it does in my world)
It anyone is upside down, it is the “northern” half, because who celebrates the New Year last? Don’t argue, just nod your head in agreement and make out I know what I am talking about.
I think I can feel the blood rushing to my head…