Not a RAAF pilot but spent a bit of time in the RAAF and went through the pilot selection process - both Direct Entry and again while I was in the RAAF. Also have quite a few friends who are current or ex RAAF aircrew.
All the info you need is on the Defence Jobs website Pilot | Defence Jobs The two paths for "off the street" entrants are Non-Graduate/Direct Entry or via ADFA. The education requirements for both are the same - Year 12 with maths/science priority. The difference between the two - one gets you flying effectively immediately (after officer training), the other gets you flying in 3 years after completing a degree.
If I was going to go through the process again (and they came up with a way to resolve colour perception issues!), I'd apply for both ADFA and Direct Entry towards the end of Year 12 - this gives you a bet each way.
A few things to remember going down the Defence path - you need to sign up for 12 years (not a big deal at 18 but something to be mindful of), the focus of military aircrew is less on being a pilot and flying an aeroplane and more on operating a mission platform and completing a mission. Know what you want to fly and why - this is critical if you want to fly fast jets - if fast jets is what you want, make this your life's mission as I doubt there is anything in life as competitive as getting a fast jet seat. Also, as a military pilot you're an Officer and military leader first, pilot second. This is even more so for the Army and Navy where you're a soldier/sailor first. Oh, and these days you don't apply to be an Air Force pilot, you apply to be a Defence Pilot, you state a preference for the service you wish to end up in but the ultimate decision is Defence's - they'll put you where they need you.
Hope that helps some.