Mate,
The pay is the least of your worries. Rest assured probably a lot more than your civvie mates. Unless they are driving big trucks in the Pilbara.
As far as the assessment day is concerned learn everything you can about the role, the P3 and the RAAF in general. A key hint, the primary role of the P3 is not SAR, as some have answered in the past its just the most publicised.
As Rogan82 points out if you are going to get airsick, it will be in a P3. You wont even go near an aircraft till the final months of course, it is something you need to be prepared for and at this stage there is only one other aircraft type AEAs can be posted to and that is after a tour on P3s.
Getting airsick in itself is not an issue most P3 aircrew get sick at some stage, its whether you have the capacity to work through it. Unfortunately its not something they can test for during selection.
If you get accepted, be prepared for a lot of study in your own time during training.
Good luck