Your brain can get worn out when you exercise it, just as any other part of your body tires from use. Practice and use of proper meditation calms the nerves and allows you to reset and refocus. To get a sense of how you can control your stress with meditation, utilize biofeedback, or the practice of measuring your body's biological responses. Monitor your pulse to music that you find soothing, find the artist or song that works best, and remember that song when the big day comes.
Classical conditioning at its core works when all else fails. Rewarding yourself for successful pre-test scores or just a hard night of studying can make a psychological difference when preparing to bring it all together. Board tests are meant to be daunting because they are testing you on advanced material, material which can not be learned nor memorized in any short period of time. Set short-term, attainable goals for yourself and be sure to pat yourself on the back when you reach them. The motivation required to study to the extent needed to succeed on a high level test like the HSC examination.
Study groups are a way to share the psychological pressure of regurgitating a year's worth of work in 180 minutes. Much like support groups, study groups share a common bond of a pressing issue at hand- this being the advanced biology exam. Becoming close to people who are going through the same stress as you can be comforting. Stress can take a tolla on a mind if not relieved, so don't be afraid to share your thoughts.
No goal is attainable with out a sense of confidence. Find it, harness it, and cherish it because if you lose your confidence, you can succumb to stress. Emotional breakdowns are not uncommon when preparing for any big test. It's how you bounce back from emotional troughs that determines if you are really ready. Proper preparation and support is mandatory for your success.