All the best for the assessment centre flying kiwi-its a full on day but really interesting at the same time.
I'm a new kid at airways (only rated last april) so the assessment centre is still fresh in my mind. 1Charlie has pretty much hit the nail on the head, and the tricky part about the sure select product, its bloody hard to prepare for the day! All I can say is go in, give it your best, and be open with the guys interviewing you. They're all good buggers (at least the ones I had!!!).
One extra to add, have you visited/considered visiting a tower before you do the assessment centre? You do get a quick trip up chch tower at the end of the day but its always good to have a chat with some of the local controllers, and see how much you like the look of the job etc.