JinXi, Melbourne only uses the 7.0 GPA system for Medicine entry. Otherwise, it's just straight averages (out of 100 obviously). If you want to get in the JD program on a CSP place you will want to be getting above a 75 average. If you got 99.00+, and you get 75+ GPA, you will get a guaranteed full-fee place (~$100,000) so you don't have to take the LSAT or put in a personal statement. If you got 99.90+, you are guaranteed a CSP place (~30,000 if I'm not mistaken).
Apparently, a ~78 GPA and a ~160 on the LSAT will net you a CSP place at the Melbourne Law School. Its getting quite a bit more competitive as the years go on (Melbourne is now ranked #8 in the world for law!) meaning that the cut-off might go up for a CSP place. If you get below the cut-off for a CSP place (which would probably happen with a 75 GPA), you will be offered a full-fee place. What you do with that offer is up to you.
TLDR: Don't aim for the minimum. Strive to achieve the best you can so that you can guarantee you CSP place at MLS' JD program.