What kind of interview questions do recruiting ask?
At the moment assessment days aren't that bad. They revamped the recruiting process in 07/08 to make it faster and less painful on the individuals. I like many remember the good old days where you'd spend an entire day at DFR and only have about 2-3hrs worth of appointments.
The medical component is rather simple, just making sure you're physically up to the challenges of training. The psych just wants to see if you're not crazy... However do not be afraid if they try to push some buttons. They don't want to crack you but they want to understand if there are any external forces that will create problems for you during training and the rest of your career. Answer honestly with a level of maturity expected of your age group, if not more so.
The actual Defence Interview varies from person to person. You'll meet with a military member usually Warrent Officer or Captain in rank (service equivilants). They will quizz you on the career you are about to undertake. Know relevant things like training locations, length of courses, (i.e. Flight training begins at OTS or ADFA, (time dependant on which stream you do), BFTS 6 Months and then 9 Months 2FTS), and possible future postings after completion of training (F/A-18 at 3, 75, 77sqns located in Tindall or Williamtown).
Also learn up on tri service things like the current personal weapon operated by the ADF, Fitness standards etc etc. They will ultimately show that you are keen and know what you are getting yourself into. Which is what they want to see most of all.
Hope this helps, Recommended sites are obviously recruiting website, and the various services aswell. Its good background knowledge to have and will put you in good stead.