It depends on the degree.
Quite a few science/engineering degrees are very much the same in first-year, which means you'll be doing the same subjects regardless. If you're not sure what subjects you want to do, you can go to the lectures for all the subjects involved for the first week or two (even if you're not enrolled in them) just to get an idea as to what they're like, and then quickly change your enrolment before the HECS date (if you decide you want to change it, that is).
You have a few weeks leeway before you're locked into the subjects for the remainder of the semester.