Unless you are absolutely gunning all your units, I wouldn't recommend picking up extra UoS during a semester, especially if this is your first year at university. Often you have to go to the Dean of your faculty for permission to do extra units and to my belief, they will reject any approach by a first year student in Semester 1.
In fact 24cp, is not the minimum amount of credit points required during a semester. Anyone can do less than 24cp, but after a threshold is passed (can't remember what it is), you will be deemed a part-time student so HECS for those units are void.
Leave the units you have chosen and see how you go for first semester. Don't consider doing more than that until you get your results for Sem 1.