Hey, mate.
You can overload if you want to, and do five courses a semester, but you have to talk to somebody in the system to do so and you better have good reason and very good marks to justify you doing so, I think.
I've read that the diploma of languages at UNSW is actually worth 48UOC, so that's eight courses (
http://intlstudies.arts.unsw.edu.au/programs/diploma-of-languages-71.html). Considering that you need to do nine courses to major in a degree...
Because you've done French Extension, you'll be doing Professional French A and B, then you're going to be doing five courses of French texts, film, or culture and then a capstone course that encapsulates everything at the end. Some courses won't be available due to a lack of enrolments or for some other reason. For example, the course on 'French Texts' isn't offered next year (ça m'éneeerve!), so you may have to do some French course such as 'French Cinema' to fill the requirements for your diploma (the course is in English though) that you may not want to do. I don't know if you want to do French courses at HSC Advanced/Extension English level in French (Jean de Florette!)! Also, just to let you know, French isn't offered at Summer School.
Could I ask you though, is it really worth it in the end, dude? Why not just go on exchange in France or some other Francophone country for a semester or two? You'll be practically fluent after that experience and you can put that down in your résumé. You'll also learn more French in a more efficient way than you'll ever learn at UNSW by doing this, and you'll graduate sooner. But this is just my opinion.