There are different application schemes for Australian citizens and for international applicants:
All Australian passport holders must apply through the state run application system where the university they want to attend is located. Each Australian state operates a Tertiary Admissions Centre (TAC) or a University Admissions Centre (UAC).
International students can apply directly to the university, or they may have to apply through the international section of the TAC or UAC. Check each university you are applying to.
UAC: Universities Admissions Centre – www.uac.edu.au
Victorian Tertiary Admissions Centre (VTAC) – www.vtac.edu.au
Queensland Tertiary Admissions Centre (QTAC) – www.qtac.edu.au