Even if you did get in i think you would find it extremely challenging. If you're getting in the 50's for the ATAR your marks at uni would be atrocious unless you really changed your work ethic, and i mean really, really changed it.
If I got into the B Design, I'd do well. I'm good at graphics and all that. Won my fair share of contests.
Also, my exam marks were all high 70's. Almost 80's, just throughout year 12, I didn't do too well because I didn't really care, and then it kicked in I was in the HSC about 2/3 of the way through the year. So bad assessment marks brought my HSC marks down to low-mid 70's.
EDIT: Oh, also, the subject that I need to get into the course I did pretty well throughout the year and in the exam in.