Make sure you take either the 891 or M50 if you are getting on at Central. The walk will be terrible if you somehow manage to get off on Anzac Parade.
If you go to Google Maps and look at the street view. For the first couple of weeks, people will be actually lining up on Elizabeth St (i.e. passed the second bridge tunnel and around the corner), or go deep into the park some 50-70 metres in.
The wait can be something like 10 minutes to 20 minutes. It's pretty bad.
If you really don't want to wait, then consider getting off at Town Hall and catch a bus from there, or go to Railway Square and see if the 891 stops there (some do, most don't) or take another bus. If you're there early, then maybe standing in a long line isn't your thing, so search alternative routes to UNSW.