As there has never been a B757 on the AU
register, then I assume it does not have Oz "type certification".
If you wished to operate one on the Oz register you would have to apply for this certification. It could certainly be done, but it would not be a trivial exercise for an airliner and would require Boeings assistance in suppling documentation to CASA. If they would be willing to do that at this late date I do not know, as there are no new 757s you aren't spending money with them.