Last year the HSC Chemistry B6 cut-off was ~76 and for Physics B6 cut-off was a bit higher than usual at ~83 lol.
For 99, for Physics you needed about ~97, for Chemistry about ~95.
To get something like 99 you need 95+. If you got into the low 90's, then you can get something around 95-98.
Marking is considered harsh because if you accidentally slip up and forget to say one word, then you could lose a mark.
Also, if you write a few words that contradict the rest of your answer, then you could possibly get an instant zero for the question.
Its actually technically not harsh, but rather they have strict guidlines which have to be followed, and they are followed word by word. Hence, the key to increasing your potential to do better is to read through the marking guidelines for various questions and to read through the notes from the marking centre for each question too and stuff like that.
So like do your past papers, but also make sure that you read up on that question through the marking centre notes and marking guidelines so that you know what you are doing wrong and what you need to improve on.
All the best