Well, you'd think if your Atar was so important to your parents, and through them, to you, you'd actually put in the effort in order to get that 90+ atar.
It's not too late right now, as the HSC progresses, the worth which each exam has increases, so you've got right until the last tests which are, btw 50% of your mark for each subject, to make the change.
If all fails and you end up with an 60,70 or even 80 atar, you can still get into PLENTY of courses in some of Australia's top uni's. You're parents desire for you to get a 90+ atar is probably a result of their experiences in asia leading them to believe that only 90+ atar people are able to get into a good uni, which simply doesn't apply to Australia the "land of opportunity". Just make sure you've applied for some course at Sydney of NSW uni, or wherever you want to go, and you can just tell them "hey, I got __ atar, but who cares because I got into ____ uni anyway".