Generally speaking, you can compensate for unfavourable performance by performing favourably and to the best of your abilities in your remaining school-based assessment tasks/exams as well as your HSC exams. Assuming there is a significant percentage that is yet to be completed, favourable performance in your remaining internal tasks will allow you to improve your ranks across your subjects, placing you in a better position as you approach your HSC exams and ultimately maximising your chances of achieving a good Assessment Mark (i.e. the mark that contributes 50% of your HSC mark in a particular subject and reflects your internal performance). Similarly, given the fact that the HSC exam of a particular subject contributes the remaining 50% of your HSC mark means that you have a significant opportunity to turn things around in your favour by performing well in your respective HSC exams, resulting in favourable Examination Marks and ultimately, favourable HSC marks. Consequently, you will hopefully be able to achieve your desired ATAR.
I hope this helps!