This is something a lot of people ask about, so I'll write this once here and not have to worry about it again.
The GPA (Grade Point Average) is an important part of the application process. Generally you'll have to have above a certain level (hurdle). Some universities will also use your GPA as part of the admission criteria, rather than just using it to eliminate unsatisfactory students.
GPA is calculated by
GEMSAS for the universities in the consortium. It's a rather simple process.
Your most recent 3 years worth of subjects (300 points / 24 subjects) are used to calculate your GPA. They are arranged into 3 "years" worth of subjects that GEMSAS calls "Final", "Final minus 1" and "Final minus 2". I find that annoying so I will use GPA1/2/3 for ease of understanding. GPA1 is your first year GPA etc. Subjects are allocated to years by counting backwards - your most recent 8 subjects are all in GPA3. Your next 8 are GPA2 and your first 8 are GPA1. If you've done more than 3 years of tertiary study, only the most recent 3 are relevant. If you have overloaded a semester, subjects will be arranged to maximize your GPA.
Once you've allocated subjects to GPA years, you need to convert your marks to the GPA scale. GEMSAS uses a 7 point scale and conversions are as follows (if your university is crazy and you don't receive marks but only grades such as HD/D/etc. you will need to look up the conversion for your university on the GEMSAS website)
Unit score - GPA grade
>79 - 7.0
75-79 - 6.5
70-74 - 6.0
65-69 - 5.5
60-64 - 5.0
55-59 - 4.5
50-54 - 4.0
45-49 - 3.5
<45 - 0.0
You average the marks for each GPA year to create three numbers - GPA1, GPA2 and GPA3. You then weight these as follows:
 + (2*GPA2) + (3*GPA3))
and divide by 6 to calculate your overall GPA.
EG: To calculate a GPA1 year with marks of:
75, 74, 76, 85, 90, 92, 60, 65 will become (respectively) 6.5, 6.0, 6.5, 7, 7, 7, 5.0, 5.5
This averages out to 6.3125
Assuming GPA2 is 6.5 and GPA3 is 6.4, the
overall GPA is 6.41 (you work it out via the formula above).
This GPA is what is passed to universities for use in your application. If your degree is incomplete at the time of application, GPA3 will obviously be underweighted, thus placing an emphasis on GPA1 and GPA2. Your first semester results for GPA3
will be included if you attend an Australian university. Don't slack off.
Any questions or anything I've forgotten, post below.
One key point: Yes, getting 100 makes no difference to getting 80. No, this is not unfair. Do not complain about it in this thread.