About the TFN, not sure if it's the same at every uni but at least at UNSW you can provide/update your TFN at any time. I would do it before term starts though just so you can get HECS and stuff organised.
Good luck everyone! I'm sure you guys will do great :) Remember the HSC is not everything, and you can have success regardless of how well you do in an exam. That being said all the best in achieving the marks you all want!
Trimesters are certainly full on but if you can time manage well, you'll be fine. Personally don't love them because you always have things to do, and the short breaks. UNSW certainly has the biggest engineering faculty, but as @carrotsss mentioned above, probably wouldn't make that much of a...
You just have to be good with time management - and don't take flexi week as a break BC if you don't do any work you will 100% fall behind. Otherwise, it's really not that bad.