GPA varies quite a bit between universities. Some universities use 7 point scales and some use 4 point scales, even between them there are differences. Macquarie treat HDs and Distinctions the same, UTS use 1.5/2.5/3.5/4 rather than 1/2/3/4 etc. I believe 4 point systems are the norm in the US and 1/2/3/4 is the conventional weighting. A 3.0 GPA under that system would be a distinction average.
The problem with using GPA is it essentially rounds down your marks as it is based on grades rather than marks. WAM is just an average of marks, so it's possible for someone to have a pretty high WAM but just like a credit average according to GPA. Additionally GPA cannot distinguish between a person who just got HDs in all of their subjects and a student who got full marks in all of their subjects - that is ofcourse a hypothetical situation but people do graduate with "perfect" 4.0 GPAs.
The number you are looking for is probably 75 as that is what a distinction average (3.0 GPA) would be under WAM, but in reality a distinction average student under the GPA system would likely have a wam of closer to 80.