Most people who sit the olympiads are only doing 1/2 in their subject, with the possible exception of biology. That said, almost everyone from NSW and WA doesn't accelerate their subjects, and even if they did, the NSW curricula for chemistry and physics are ridiculously bad.
The most important thing to remember is that the NQEs ultimately test reasoning ability. You can honestly get into any of the subjects with a limited knowledge of the field - if you know the basics, you'll be fine. As the cut off for chemistry is generally about 35-40% on the NQE, if you can get the multiple choice and simple questions that require knowledge you can get in, but you can still make it if you're still brilliant... it's just harder. Just remember that the work at the summer school and the FSEs are MUCH, MUCH more difficult than the NQE, so if you get into the summer school for chemistry based solely on knowledge you might not do amazingly later in the program. The physics paper didn't require much knowledge - if students knew conservation of momentum and how forces act, it was doable.
Oh, and guys, keep in mind that the olympiads are hard, irrespective of how old you are. They're designed to identify the top handful of students in the country. Not your class, not your school, your suburb or Melbourne or Victoria - but Australia.