As far as I'm aware, this should never happen- if you get a raw mark above 50, then you won't be scaled down to a fail. However, that might not always be true of higher grades- some Usyd faculties do have policies that regulate how many students can get a certain grade (for instance, I think law requires that no more than 40% of students can get a distinction or above).
So, in short, you won't be scaled to a fail, and scaling down in general is comparatively rare in most courses.