I was asked about military training at ADFA and then employment training afterwards. Know what you're potentially getting yourself in to; know where you will train, how long it will take, what you will learn and what is required on your behalf.
I had both RAAF and Army as my preferences and I was asked questions on the aircraft from both forces, all other questions were applicable to all of the forces (aside from the training questions).
In short, the best way I found to tackle assessment day is to know what you're going for and know why you want to do it. So, as has been said a thousand times, study the defence and RAAF websites and know what the job and the training entails.