I did my HSC last year, but hopefully this is still helpful!
I am most definitely an early bird, so I found my productivity dwindled by the time it hit 9pm (unless I was on a roll, in which case I could work until 11). This is my schedule:
3pm-- Arrive home, catch up with family, take a breather.
3:15--5:30-- Study chunk 1. Completing in order: Homework, assignments/projects, independent study/practice exams. (It's basically Helen's urgent/important classifications)
5:30-6:30-- Dinner and evening walk
6:30-8:30 -- Study chunk 2. This time is mostly for writing study notes and whatever else is important (usually project work or practice exams). If there's nothing on, I'll give myself the break.
I consider myself more self-disciplined, so most days I can stick to this schedule pretty well. The Pomodoro technique (25 mins work, 5 mins break) helps a lot if you tend to lose focus quickly.
It gets easier with practice. Good luck!!