I dont think it happens if you "fail" but it depends what you mean by "fail"?
People who got N awards last year (i.e didnt complete the course) got to start year 12 courses for the ones they completed but had to do pathways with the others, so they had to redo year 11 for that course.
I dont think it matters if you get under 50% in a course but it does matter if you dont do all the assignments
Dont worry so much, your school would let you know if anything like that happens