If your internal performance is average and your HSC exam performance is outstanding, then your Assessment Mark will be average and your Examination Mark will be outstanding.
The Assessment Mark is the mark that reflects your internal performance and contributes 50% of your overall HSC mark in a subject. It is essentially determined based on your rank in a subject as well as the Examination Marks (i.e. HSC exam marks) achieved by your school cohort. To maximise your Assessment Mark in a subject, you should ideally aim to place as high as possible within your school cohort by performing at the highest possible standard in your school-based assessment tasks/exams. If you perform at an average level in your assessment tasks/exams, your rank may not end up as high as you may hope, which can consequently limit your Assessment Mark.
On the other hand, your Examination Mark in a subject is determined according to your own performance in that subject's HSC exam. As such, if you perform well in the HSC exam of that subject, you will receive a favourable Examination Mark (which contributes the remaining 50% of your overall HSC mark) for the subject. Unless your internal performance is of a similar standard, in which case you may receive your own Examination Mark as your Assessment Mark, your Examination Mark will not affect your Assessment Mark. Instead, it will be used to determine the Assessment Mark of another student whose internal rank is consistent with your Examination Mark.
For example, if your Assessment Mark was 75, and your Examination Mark was 95, your overall HSC mark for the subject would be:
I hope this helps!