what on earth is average? you mean like, in the state?
The thing is, you may get 82 in your HSC exam, but if a lot of people got 82 in their exam, then obviously that shall be the average and will throw your UAI in a different area. Do you get what I'm saying? It's not as easy as an "average"
by average, you mean "not as well as you hoped for", then it really depends on how well you do in your other subjects. It's okay, you shouldn't be stressing heaps.
If you're saying this because there is one subject that you're not as good at compared to your other subjects, well, you're on holidays... try doing some practice essays or go through your notes, you still have time...
all you can really do is your best