The information that was provided above constitutes a good summary. Essentially, your overall HSC mark in a subject consists of two marks, which are the Assessment Mark and the Examination Mark. The average of these two marks is taken, and that figure is the overall HSC mark in a particular subject. In other words, the Assessment Mark and the Examination Mark each contribute 50% towards the overall HSC mark.
An important element that is used to determine the Assessment Mark is a student's overall rank in a subject relative to their cohort. This rank is determined through the student's performance in their school-based assessment tasks/exams. The better their performance, the higher their rank, and the higher their Assessment Mark (and vice versa).
To answer your question, unfavourable performance in an assessment task will have a negative impact on your rank. However, given that there is still a significant percentage of school-based assessment tasks/exams that is yet to be completed, you have the opportunity to improve your rank throughout the year. Many students do not start year 12 with good results, although they are able to make a comeback throughout the year. It is also important to note that because the Examination Mark contributes the remaining 50%, this makes the HSC exam just as important in terms of determining whether you can achieve a band 6 in a subject. You should therefore ensure that you thoroughly prepare for your HSC exams when the time comes. Coupled with favourable internal performance, favourable HSC exam performance will help you maximise your chances of a band 6 overall.
I hope this helps!