When I was in Australia, I was planning on attending Oxford, however, I got rejected. It might have been a bit different for me because I have German citizenship, but here are a few points you might want to look into:
- are you positive that you can archive an UAI of 99 (I don't know how much of a gambling the NSW system is / whether you're doing the IB)?
- check out the admissions web pages. A lot of useful information about admissions can be found there.
- (at Oxford) some subjects require written work to be submitted / an entrance test. Make sure to know and be prepared to do whatever your subject requires.
- maybe you want to order an prospectus from the university. I orderd one to Australia and the cost was 5 pounds (about AUD 12, for Oxford that is again). It was really worth it because I was actually able to just look at it without a computer.
If there is anything else you'd like to know, don't hesitate to answer. I'd be happy to answer it to my best knowledge. Good luck with your application.