That is crazy if I'm interpreting the guideline correctly. So a 'B' is a 7 for most Australian unis, while even a B- is a 7 at places like Melbourne?? That would make my old 3.2 GPA somewhere around 6.9 at Melb (allowing for two grades below B-).
It used to be (at UQ, USyd, and Flinders at least) a straight-forward math calculation, e.g., a 'B'=3.0 ==> 3/4 = x/7 ==> x = 5.25. Then weighting of year GPAs simply ( (Y[x] x 3) + (Y[x-1] x 2) + (Y[x-2] x 1) ) / 6 (x being the most recent year).
There seems to have since been a bit of grade inflation to put it mildly...