Depends quite a lot on the subject and of course variance year-to-year.
I'd say STEM gets a lot more resits & fails, just in the nature of if you cant answer the questions you get zero, even if you attempt some questions if you dont know what your doing you might as well not answer, hence theirs always some students who have poorly prepared for exams and hence its easier to flop. That said generally marks are fairly linear to paper marks so its much easier to score 80+%.
Im guessing like 10-25% resit in STEM but i think less than 5% have to re-do the whole year.