If you are on a Youth Allowance or Austudy payment Centrelink will only pay you for the usual length of the degree plus one semester. So for a three year degree, you can only be paid for 7 semesters. This calculation only considers when you were enrolled, not whether you were paid. Part time semesters (fewer than 18 credit points) are calculated as half a semester.
Centrelink will normally write to inform you that they are going to stop your payment a month or so before the end of that semester. Do not ignore this letter. Sometimes Centrelink will miscalculate your allowable time so make sure to check that they have got it right.
If you are doing Honours, your calculation will be slightly different. For a stand-alone Honours degree you will get 3 semesters for the Honours component of the degree. Where Honours is embedded in the degree you will get 2 semesters added to the entire time allocated for the degree.
If you have failed whole semesters because of illness or misadventure you may be able to ask for those semesters to not be considered in their calculations. If you have only a few subjects left for your degree there may be a way to change to NewStart Allowance.
Talk to an SRC Caseworker about any of these situations.