If you have a satisfactory excuse then you will still be given a mark. If you choose to simply not turn up then you get zero (and it does appear on your HSC record - have had students do that, particularly with their final exam and simply decide they can't be bothered to do another exam believing that their other exams are good enough anyway). It isn't removed in my experience unless the student formally drops the subject before the exam.
"If you are absent from an entire examination and lodge an illness/misadventure application that is declined by the BOSTES, you will not receive a result in that course and that course will not appear on your BOSTES record. This could mean that you become ineligible for the award of the Higher School Certificate."