Thats not necessarily correct. You can come last in your class and still get a decent mark. Considering that your actually hsc mark is worth 50%, if your moderated assesment mark is something like 50, and you manage to pull of a miracly and get 90, your still looking at a mark of 70, which is much better than 50. And coming last in your class does not mean you are going terrible, eg lowest mark in my class for ext maths (non-selective school) was something like 60%, and thats not disastrous. Just try your hardest and you'll get the marks you deserve. Maximising your position will help in maximising your marks.