"on hold for upcoming assessment centre" is all sweeet. That just means you are through to the next stage and they will send an email at some stage with various dates to choose from for the next assessment, first in best dressed!
If you were nervous for the phone interview look up how to do a STAR interview and sit down with someone and get them to ask you some questions.
Most importantly be yourself, enjoy the day. Best of luck, it is a fantastic career (In NZ with Airways anyway)