Don't be scared nicky! First of all, don't look at your assessment mark, look at your assessment rank. 70 in assessment marks does not indicate you'll get 70 in a UAI. In fact it really gives no indication of anything. Instead, look at what subjects you are doing and what your class rank is to get an idea of what you might expect.
As for giving up hope. No! It's quite possible to significantly change your marks between now and the HSC exams. If you work hard you'll boost them up. If not then they can drop down just as far.