Well if you haven't got onto applying for early entry I would do that now. Entry closes earlier than normal applications.
Interview advice...even though you sent them a portfolio, bring your own. It shows organisation and if you need to explain anything you can have it in front of you.
Choose essays that are well written and marked well, but maybe also look for something where "your voice" really comes through. Something that isn't just a good essay, but one that demonstrates a little about you. I am not sure if HSC students have to supply an essay - I needed to if I wanted to do it because I was an ACT student.
Think about everything that you give them, including recommendations and be prepared to answer questions about them. Like why you chose those classes, why you responded to the essay question like this etc.
They are going to ask you why you are applying for the applicable course (probably) and try and give a good reason. Don't just say, oh I thought it would be fun, or they pay good, or that you have nothing else better to do. Really have it set in mind your goals, and your motives. Even if they change later on.
Remember that the interviewer is not judging you, they are judging the facade you put up. Shake hands when you enter but do not intrude in their personal space. Don't fidget, when you smile make eye contact. Don't smile unneccisarily. Don't laugh needlessly, that weirds people out. Don't cross your arms or hunch or look down, look calm, open and present. Listen to the questions that they ask and answer them fully.
Just prove that you are going to do well in your exams and that you would be an asset to their university.
They are my tips.