The topic for HSC composition is "Australian Music of the Last 25 Years", and your teachers are likely to tell you this basically gives you carte blanche over whatever you want to write, since some Australian composers Have explored many different traditions. But if you want the best marks, then your compositions will reflect the ideas explored by modern Australian composers such as Wesley-Smith, Boyd, Sculthorpe and Smetanin (the last especially for those planning to apply to the BMus (Comp.)).
I hate to sound like an advertisement, but the best teachers of composition are not your high school teachers, but composition students a few years out of the HSC themselves. We're a little more in touch with what's going on in the Australian music scene.