No, not really.
It's not really 50% of your assessment mark, but 50% of your moderated assessment mark, which can only be determined after you've sat your HSC exams.
You could be sitting in last place on 40% raw assessment mark, but still get a band 6 for that course so long as your cohort does extremely well in the HSC, where the whole cohort gets an aligned mark of 90+.
Then they grab half of your moderated assessment mark and half of your HSC exam mark, combine them together to get your final HSC mark for that course.
But for the ATAR, UAC uses your moderated assessment mark and your raw HSC mark to determine your ATAR rank. So with a 90+ moderated mark and whatever raw mark needed to attain a band 6, your ATAR will still be high, in this case.