A few days ago I was reading this book at the bookstore (instead of buying it, heh), and something that really struck me was the author's perception of motivation.
Basically, what he said was that motivation isn't something that you wait around for all day hoping that it's going to come knocking on your door.
Motivation doesn't come first, and study second. In fact, it's the other way around. Just do it, was what he said. Do what you need to do, and motivation will come later - and that will push you to continue on.
That reminds me of a time at the beginning of year 12 when I just didn't know whether I was going to do well that year or not. Because I didn't know, I just studied because that was the only thing I could do. To my surprise, I topped biology and chemistry, but failed very miserably in maths. Both of these outcomes gave me the motivation to continue on.
Take it one step at a time. Think about the next exams that you have. Prepare for those. Don't wait until you have to fail for you to realise you immediately need to fix your habits of not study. You're not spending your time doing anything else anyway, so make use of it.
I think something very important is to have a reason to study, perform well, or mediocre. What are you doing the HSC for? Do you want to go to university? If so, what course do you want to get into? If you don't know, it's time to research now. If you still don't know, i think it's important to still do the best you can to ensure you're able to get into a wide range of courses. Do you have an ATAR goal? It might be worth it have one. So that you are working towards something. Always have something that you are working towards.
*edit: I just saw in your sig that you want to pursue a career in business/teaching - it might be worth it to find out what the ATAR requirements are for these courses at the uni that you want to be studying at.
Start now. Start today, not tomorrow. If you tell yourself that you're going to start tomorrow, you'll never start, trust me.
Anyhow, I hope this has helped you. Good luck on the HSC and I hope that everything works out for you. If you have any other concerns, please don't hesitate to ask us.