You pay based on how many subjects you do. Each subject falls into a band category, and each band you have to pay a different amount. You will need to add these up for the duration if your degree to find the final cost (which won't include union fees, etc)
Hybrid Hunter , I dought it, read the example in the HECS red booket, I dont think you can pay more then 6200 on HECS per year for a single or dubble degree.