What a WAM is
Australian universities calculate a weighted average mark from the actual percentage you scored in each unit, weighted by that unit's credit points. A 78 in a six-credit unit alongside a 64 in a three-credit unit produces roughly 73.3, and that figure carries the same information a percentage always did. Institutions differ on the details: some weight later years more heavily, some exclude first-year units, and some count failed units while others do not. Honours classifications, scholarships, and postgraduate entry are typically expressed as thresholds on this number, which is why it is treated so precisely in Australian institutions. Your university publishes its own calculation rule, and that rule rather than any general description governs your figure.
Why a GPA is not the same object
A GPA discards the percentage before averaging. Every mark is first mapped to a grade point, so 85 and 94 may both become 4.0 and stop being distinguishable, and the average is then taken over those points rather than over the marks. That mapping loses information in one direction and cannot be inverted in the other: a 4.0 could have come from anywhere in the A band, and nothing recovers which. Australia also uses four-point and seven-point grade point scales at some institutions, which adds a further layer of confusion, because a 6.2 on a seven-point Australian scale is a strong result and would be nonsense if read as a US figure.
What to do when a form asks for the other one
Send what your institution issued and let the receiver convert. Most graduate schools, scholarship bodies, and credential evaluation services publish their own equivalence table, and that table is the policy deciding your case regardless of what any calculator produced. Where a form insists on a four-point figure, ask the receiving institution which conversion it wants used, and state on the application which scale your original mark came from. Use a calculator when you want a working estimate of where you stand under a stated conversion, and keep the mark your university issued as the number you report. The thing to avoid is presenting a converted figure without saying it was converted, since that reads as a claim about your transcript.