I've never done an interview for a university course, so unfortunately I can't give you any specific advice. With interviews generally though, you should treat it a bit like a test, which means that you need to 'study' as it were, through preparation. Think about times in your life where you've shown certain skills (good organisation, communication, times when you've had to be flexible or deal with competing interests, etc.) and bring those to the interview. Look over your CV, and also think about class experiences, group work, that sort of thing.
Also, think about why you want to go into the field that you are interviewing for, because that's bound to come up. Don't give silly reasons like money, or as a launch pad into something else. They are probably quite persuaded by personal anecdotes, but if you don't have one, just talk about how enthusiastic/passionate you are. Additionally, if you've wanted to be a doctor/physiotherapist/dentist/etc. since you were very young, it's fine to say that, but you should also indicate that you've reflected on your career goals sine then (you don't want to sound like you're only doing this course because it seemed like a good idea when you were 6). If you have future goals, where you want to take your degree, etc., it's definitely worth mentioning as well.
You may also be asked why you specifically chose Monash to do your course. Don't say 'the Melbourne Model'! Come up with some reasons why you think Monash is the best place in Australia to do this course.
I doubt that they'll ask about your weaknesses (I've never been asked that in an interview), but if they do, talk about how you've overcome them or you have taken steps to overcome them. In fact, any time where you mention a problem you've had in your life, talk about the solutions you had.
Ummm... I think that's all the advice I have. It feels strange, because as I said, I have pretty much no experience with university interviews, but if you do have any more specific questions, go ahead and ask them. The way I see it, if you've gotten this far that reflects well on your marks and such, they're just making sure you see well-suited to the degree and to the profession, which is impossible to work out in a 15 minute interview, but you still have to do it anyway.