Instead of doing postgrad coursework have you considered doing postgrad research in that field? As I understand domestic students are exempt from tuition fees for both MPhils and PhD. I know of people who have really changed fields when going to postgrad research (e.g. going from music to animal science). And you can still do it part time.
Could I do that not coming from a science background (well not since yr 12)? I think its something I'd look at.. I like research I think I'd still want to do a foundation physics subject to start but if this is an actual option it'd be great