If you are a top performer and get solid marks for the rest of the year, you should be able to garner yourself a decent rank. Provided you perform to the same top standard in the HSC, you will still achieve a respectable though not outstanding mark, depending on the marks of the rest of your grade. Considering this may not be an overly substantial fall in your mark compared to your potential, and coupled with the fact you have other courses that havn't been mentioned and are aiming for a UAI above 80, I'd say you have an excellent chance of achieving a UAI well into the 80's.
However, this is provided you are a top, to very good performer. If you are an average student, and I mean this in no offensive way, it will hurt your chances significantly.