You are assessed on the better of your original high school ATAR, and an ATAR equivalent calculated from the GPA of any previous tertiary attempt of at least one year's duration. Since ATAR cutoffs change from year to year, there is no fixed GPA that can be quoted to you, but the conversions UAC uses give a 5 GPA approx equal to 90 and a 6 GP approx equal to 96.
If you do a unit that is reasonably close to a usyd unit then you can apply for credit.