They do kick you out for PhB :p I've had two of my friends get kicked out - one at the end of first year, as her average was low 70s or so, and one at the end of second year, as he was always borderline 80 and then got a 73 average last semester, and so they drew the line.
But yeah, PhB students usually do 3 "normal" science or arts subjects each semester (i.e. the same subjects that a BSc or BA student would take) and one Advanced Studies Course that's all researchy and stuffs.
The cutoffs are all weird at ANU this year. Everything that used to be 75 has gone up to 80 except Economics, which went up to 84. And Law up half a point? It's all very strange, as usually the cutoffs are fixed and don't change at all from year to year.