At USyd, we actually talk of a weighted average, so subjects are weighted for their relative difficulty (1 for first year, 2 for second, 3 for 3rd, ...) and also for their size (so an 8 unit subject is not weighted the same as a 12 unit subject).
My WAM is an HD WAM - that is, it is 85.00 or above.
Straight HD's - ie HD in every unit of study in every year is unbelievably rare - only two people I know have managed that, and I wasn't one of them! However, my lowest mark in any Science subject was 81, so I think I did OK.