Studying excessively during the summer break -
It's definitely a good idea to be organising/writing notes, finishing off as much homework/assignments as possible, but don't freak out if you didn't do too much work. If you're behind, or you're really uncertain about a particular subject, use this time to perfect your skills until you're as confident as you can be, however, if you're a smart, solid worker, don't get too down on yourself if you went out a little too much and didn't get enough work done.
I worked pretty hard during these holidays last year, but I didn't 'study'. I finished off extra work (that many people in my classes didn't ever end up doing, and yet many got 97+), I started doing my notes for content-based subject such as Ancient History (this was really helpful later on!) and I familiarised myself with HSC dot points and criteria. That last bit was the most useful!!! You need to know exactly what each subject requires to get the best marks. People who know 150% of the course can't write 150% of their knowledge in a 40 minute essay, so make sure you know how to approach assignments concisely, so looking at the criteria during these holidays is key. It will help you so much throughout the year.
I didn't get enough done during the summer break, and as soon as I got back to school I got glandular fever and my attendance was about 3/4 or less throughout the year, so whoever says that you need to 'study' really hard these holidays to get 99+ is being over-dramatic. Doing work is definitely a must, but don't stress out too much, you shouldn't overload yourself in this break. Relax! Go out! Have fun!