Few things you need to consider before joining up ADFA...
1) Which service will you be going for? IE army, navy or RAAF? In your service of choice, which job will you be aiming at? IE Pilot for RAAF or logistics officer for army and etc?
2) the Return of Service Obligation - your initial minimum period of service should be around 7 to 15 years, depending on which job you are going for. This means you will be spending that much time in the military after you graduate. The military is actually a lifestyle rather than a profession.
3) Are you medically and physically fit for a military career? Are you ready to be posted overseas to serve in unstable areas, eg. with Operation Catalyst in Iraq?
4) Are you a leader? Do you think you have the qualities for an officer? Cadets graduate from ADFA to be future military officers, and during your recruitment process they will be looking for officer qualities in you, and I can tell you its really tough.
5) The recruitment process - its a really long process, and you might not make through the whole thing. My own recruitment process took more than 7 months, so it's best if you start registering early. Don't wait till late, or you'll have everything stuck together during your HSC.
These are just few things of a huge list that you need to consider before making the decision, and also, every year people from all over the country go for ADFA, and theres only around 350 spots in the academy.(more specific, 160 for Army, 120 for RAAF and like 60 for Navy, although navy spots are never filled.) It is very competitive, so it will be a smart move to have a few back up plans if you unfortunately fail the recruitment process. Untill then, good luck!