Well it is as simple as it sounds - percentiles are percentiles - but whether they meant percentiles is a different story. Going by percentiles is very unlike universities as far as I know. If this is the case, I don't know how Monash and UoM could have different systems. Depending on the difficulty of the exams, those attending one of the unis would have an advantage over the other. As for what you need to get a certain percentile, no one would know. It depends on how everyone else goes on the assessments this year. If you're asking for a ballpark figure, then maybe someone would know roughly from previous experience but there's no accurate way to say really.
EDIT: And wait, so what exactly is it that you don't get? You know how to calculate your own raw overall mark, so they'd just rank that and get a percentile. You say it's not as simple as that but then you do the calculations and show us it is. I'm confused =/