CT
I wouldn't get too specific with your reasoning. Remember your goal is to be a pilot, but you also need to show that you're going to make a good RAAF Officer above all else.
Things to consider:
You want to be a military pilot, not a commercial pilot, what does the military give you that commercial won't? Teamwork? Set career progression? Stable pay/conditions? Adventure and seeing the outdoors through extra curricular training?
What do military aircraft offer you that commercial won't? Low and fast? Smaller teams of people that you can build solid friendships with and see the world? Unique opportunity to fly some of the best aircraft in the world?
You'll get the best training in Australia and potentially the world. That one speaks for itself.
Whatever reasons you come up with, rehearse them, make them genuine. Make sure they highlight the fact that military is what you want. They want fast jet pilots, not freeloaders trying to get to the airlines. Saying "it's what I've always wanted to do" is valid, but there must be something else there that's really solid, and drives your motivation and eagerness to get through the real crappy times, because you will encounter more then your fair share of them through training!
Good luck.