I've been at this program as a volunteer and I'm not sure if it's compulsory.
There is a lot of info and help given during those few days, like info about the uni, help with finding accommodation, help with how to use public transport here, info about sydney, and you can sign up for tours organised for international students of different parts of australia and stuff. Although this is all very useful, I don't think it's compulsory.
However, I think enrollment for you guys might be on one of those days too, which you'd probably need to be at so you can enroll into your course. I suggest emailing someone about it, the university, or ISSU (international students support unit) or whoever else you might have the contacts of that would know for sure.