I think that whether the depression is DUE to HSC or DURING the HSC is important -
Due to HSC - it's mainly going to be full of worries about marks, not getting UAI/job you want, losing contact due to studying, ect. Once the HSC is over, it would (in theory) be easier to recover from (note, nobody just 'gets over' depression. It's not like you tripped over a rock or something. psshh!). Unless, of course, the depression caused other areas of life to disintergrate to the point where the focus of the depression shifts.
DURING HSC would be worse, in my opinion. It's assuming that a) something else has distracted you (aka, bad breakup, death in the family,ect) OR
b) Prior Depression. This is depression a person had (long) before and would be sort of an in-grained behaviour.
Any depression, of course, there are the usual recommendations
1. Excercise. Or at least get out of the house!
2. Meet new people
3. Have hobbies.
4. Don't over stress
5. Call a helpline
And while of course all of these would work (I suppose), I'd like to ask: if you were depressed, as in truly CLINICALLY DEPRESSED with antisocial/selfharm/suicidal tendencies, then where the hell are they supposed to get the motivation to do any of these? That's one of the things we learnt in mental health 101 at school - that Depression = no motivation.
My (perhaps harsh) advice is this: Grit your teeth and bear it. HSC will not last for the rest of your life. You are in control of yourself, even when it feels like it's all lost. If it's lost, it can be found!
Note: I am not willing to state where I get my info from (whether it be from a friend/family/myself who suffered Depression or simply because I listened in PDHPE) because that's information I can't divulge on a place where people I know might find out. And apparently, someone from my school has figured out that this account is me...