95% sure it does not override the original result; crappy grades can't be removed from your transcript. Would be good if someone could say for sure though.
EDIT:
Units where a pass grade (PC or a P) was awarded
Students can only count the credit points of a successfully completed unit once. Any student
who chooses to re-enrol into a previously completed unit will have both grades included in their
GPA calculation, but only one set of credit points will be awarded for the unit. Both attempts
will also appear on the student’s academic transcript.
It is not recommended that students repeat a unit in which they have previously received a
pass grade.