yup you have to have a proxy. They need to bring
1. A signed statement from you authorising them to act on your behalf, get these off the website -
www.usyd.edu.au/su/studentcentre/proxy/proxy.html
2. Your letter of offer (the one you will get from UAC)
3. Some method of paying your enrollment fees - cheque, money order, eftpos, credit card
4. Your subject choices for 1st yr - look at the faculty handbook online, many courses have set first years so you may or may not have to chooose subjects..... dont stress, you can always change them afterwards, its not a big deal.
5. Your personal details - address, date of birth etc.
6 If you are a permanant resident, proof of the date that permanant residency was granted.
Any errors that are made can be fixed up later on, the most important thing is that someone shows up on your behalf to accept your place so it is not offerred to someone else.
cheers!