Gee, wow. That's a big ask. You are already behind the eight ball by a whole year, even in Beginners German.
Don't you have to complete the Prelim Course anyway, before being admitted to the HSC Course?
And by distance Ed, even harder if you have no one to tutor you or give you speaking practice and so on. Unless you have someone you can go to for help, or some basic background in German, I wouldn't take on German for the HSC.
But you need to decide for yourself.
There is always University where you can start beginners there and work your way through. You don't have to have studied German to enter a German Beginners course - particularly at Macquarie Uni where you can do a degree and a Certificate or Diploma of Languages (in German) concurrently.
Something to consider. But I would try and maximise your HSC marks by doing subjects you already have a background in. (IMHO)