I believe the exact matter varies from school to school. At our school, from years 7-10, anyone who fails a SAC must resit it. They then get the average of the two marks. In Year 11 however, the lower mark goes on their report, but still "pass".
Also from my experience, in Years 9 and 10, and so far through this year, if you got a SAC mark which wasn't satisfactory, in the sense it wasn't near your usual average, the teacher would let you resit it, for the average of the two marks. This rarely affects the overall rankings and scores in the end. All it does is simply make for a better report.
Year 12, I don't teachers would take the risk of changing up SAC results. However, I have heard of cases where people get close to the pass mark, that the teacher simply gives them free marks, solely for their benefit. These of course are done by a case to case bases and are meant to be "confidential"