I am going to be very honest with you.
Can you turn it around? Theoretically, yes. Is it likely? Yes and no, depending on your attitude and the circumstances at your school.
Bias is inherent within humans, there is no magic drink or pill to erase this within us. If you are the person that did very bad, and I mean, you are failing (below 50%) and the average is 80-85%, it is going to be extremely difficult to catch up. I say this parted on the concept of bias. I can sit here and reassure you that you still have many marks available in the 'assessment pool' and that you will get band 6 and be on your way to the 'riches', but to conclude at that would be highly unreasonable. No teacher will mark your assessment tasks with impartiality, if you got 30% in the one before. They will mentally note "this was the outlier, he is not going to be within the top five". Now, I know that I will probably get a lot of hate for this, but the facts are the facts. I have experienced this personally, and I have had friends experience this. The teacher marks your work with great detail, looking for any minor detail to rip it to pieces. The difference between you getting a band six and the consistently high-achiever getting a band six; the high achiever just has to satisfactorily comply to the criteria, whereas you have to produce the Shakespearean masterpiece of the 21st century.
Needless to say, what I am relating is extreme. If you are within the average bracket, you should be okay. I have seen this repeated time and time again, and I believe Enoilgam is the original creator of this - It is easy to commit to an idea, but it is difficult to commit to a process. I can sit here on my throne, wearing a robe and eating expensive fruit, and thinking about my great ATAR; but if I do no work, I am in for a reality check in late December. If you expect that you are going to have a metamorphosis from a student who did the bare minimum to avoid trouble, into a star-student who consistently studies everyday; you are not thinking realistically and need to put things into perspective.
Understand your weaknesses, work towards a remedy and I will also promote the idea of not talking to ANYONE about your study pattern. Everyone in the HSC wants to have a 'competitive advantage', often desiring to bring you to complacency (successfully so) so they can continue their study pattern and beat you. This exists, and definitely, its prevalence in my school cannot be denied; nor can the influence of such individuals on students be undermined.
If you still do not achieve your desired ATAR, there is no problem. We live in a beautiful country and there are many other ways to get into university, and the HSC is definitely not the only admission ticket. If you also notice, the most successful in the fields do not get it right the first time.
Also, please provide us with your ranks. With accordance, the more informed individuals can give you a more realistic view other the speculation that we are all inevitably responding with.
my half yearlies are worth 20-30% 
