Depends on whether you are doing Music 1 or 2.
Music 2 expects a higher standard. You are expected to perform classical pieces, rather than contemporary. I'll just guess that you're doing Music 1, as most guitarists prefer.
My main performance tip would be don't over-rehearse. I always feel (and i'm a male soul singer) that when i over-rehearse a song, it loses all its feeling. Your licks should be impulsive, and yeah, you should have the notes down pat, but if you know something inside out and outside in, it loses the emotion of someone who is in the moment of the song.
I would say that eye contact, and acknowledging the presence of a marker is great too. And look confident, don't let them know that you're nervous.
You need to get tuition for guitar and voice. I can't push this enough on you, but it will help so SO much. Having someone to say 'No, this is bad. But do this, and it will be better' is so much help. Also, if you get a good, qualified tutor, they will have taught HSC students before and know what to expect. I wouldn't a tutor neccesarily constitutes good marks, but they are a great help.
Good luck!