I was there 2007/2008, it was a great atmosphere but the fireworks were not the best in my opinion. People head to the Botanic Gardens and Mrs Macquaries (sp?) Point for the best view but they get there from quite early in the day as it gets busy. The security is strict on bringing alcohol in but there are means and ways of getting it in.
I unfortunately had to work during the day on NYE so couldn't get to the Botanic Gardens in time so headed right to the end of George St/Pitt St and sat in the raod watching the fireworks drinking cans of Bundy and coke. We then went to a club down George St after (although had previously gone there earlier and got a wrist band so we could leave to see the fireworks). All in all good times. You'll be fine to leave it until last minute plus you'll meet other people who will not have made any plans! Aussies are very laid back!