I was quite close to USYD (Moore college) for a bit last semester.
From Redfern
Walk, it takes about 12-15 minutes depending on speed. This option is definitely better getting home sometimes, due to long queues for buses.
Pros
Exercise is good.
Quickest way.
Cons
Not good if coming from Airport/East Hills line or Blue Mountains. Less than ideal for Wollongong as well.
From Central
Buses to City Road run from Railway Square Stand L to Newtown every 5 minutes (422, 423, 426, 428 and the M30 are such routes)
Buses also run from Railway Square Stand M to the Parramatta Rd entrance (M10?, 436, 438, 439, 461 etc.)
Pros
For those who find walking challenging
If you have travelled long enough, the bus only costs $0.07 !!
Cons
Buses can be crowded at key times and can be delayed due to traffic. it can take 15-20 minutes sometimes to get a bus from Central to campus, and that does not include time waiting nor the time it takes to travel from say Platform 24-25 to the bus at Central.
Summary:
If you are travelling and your train stops at Redfern, and you don't mind a walk, then that is your best option.
If you are travelling to/from T8 Airport Line or from Mascot/Green Square outside of peak hour, then best to change at Central.
Late at night, Redfern is a little bit dodgy, so I'd also recommend after 5pm, getting a bus to Central is safer as well, then walking the back streets of Newtown.
If you are travelling on an intercity train or another long distance destination, if you don't mind paying an extra 7cents then the bus is good.