Throughout the year just maintain your notes. Ensure that you are up to date with summarising stuff from the textbook etc. At this point, don't bother memorising the stuff.
When it gets to crunch time before the HSC (a couple months before that is), you need to start memorising (sadly that's what HSC Business Studies requires). The syllabus is the first thing you need to memorise. If you look at the mutliple choice and short answer questions from past HSC's, each question is directly linked to a syllabus heading and if you know what is under that heading, then you know exactly what to talk about or lock in as your answer.
The problem that many students face in the short answers is not knowing what to talk about. The concepts in Business Studies are very, very easy. As long as you know WHAT it is that you need to talk about, almost any student is capable of stringing together a few sentences that will get you over the line.
For extended responses, memorising the syllabus is going to do you wonders - it becomes your essay/report planner. The question will directly relate to 3 or 4 parts of the syllabus and if you are smart, you will jot down every heading that is under these parts on a blank piece of paper. This ensures that you don't miss anything (which is exactly what the HSC markers want to see). Then, this is where your notes (that you have been compiling throughout the year) come into play. You should have the knowledge to be able to spin shit basically on any part of the syllabus - your notes will ensure that you are able to do this if you keep reading / writing / understanding them.
If worse comes to worse, and you haven't prepared your notes by HSC time, then this is the perfect resource:
http://www.jaconline.com.au/businessstudies/book2.htm
I used these notes together with my own notes (which were basically copying out the textbook in a summarised format) as well as listening in class to achieve my HSC mark of 94. To be honest, I did about half an hour of updating notes etc per day for business studies. I didn't bother doing any questions from the textbook etc. The two months before the HSC was spent memorising the syllabus and practicing past HSC paper questions.
To the OP: If your getting 70s without studying, you have nothing to worry about. It is very easy to snap out a Band 5 to 6 in Business Studies. A little bit of work will get you there. Good luck.