raymondli, you're probably not going to get many answers here, these sorts of interviews with UNSW specifically are new as far as I'm aware so very few if any people here would have gone through them. Go talk to the Uni directly, you're planning on dropping a lot of money with them and they want that money as well so they should be more than willing to help you out and explain the process a bit better.
And as others have said, if you're stressing a lot about the interview process then...perhaps have a good hard think about the job you're looking to get into!