New year's eve is a bit zoo-like, so you need to decide if you want to see the fireworks or not. They are undoubtedly spectacular: I've seen lots of firewords displays in various locations, but none has ever compared with the Sydney ones on new year's eve, or even come close to it - the "theatre" (i.e. the harbour) is a marvellous showcase for the display and there's a sense that, somehow (illogical though it probably is, and the cynics will no doubt scoff), you are witnessing something quite special.
Assuming that you have your accommodation sorted, you need to decide where you want to go to view the fireworks. Inevitably you will need to get there early and spend quite a lot of the day there if you want to see both the harbour and the fireworks. (There are places from where you can see the fireworks but not the harbour.)
The next two days - 1 and 2 January - will be much calmer: you are in a large city that is "on holidays" and in a party (or recovery) mode, with lots of visitors as well as locals moving around, but the frenetic pace of the previous day, and the transport difficulties, will be over.