Surely this about who has a duty-of-care as the senior supervisor on site, not when the regs indicate the PIC's responsibility for a flight starts and stops?
While the PIC may not be legally responsible under CASA regs, company policy and Common Law would require they take control of a situation in the absence of another authority.
On the ground with the door open, that higher authority could be the airport duty manager, police, border force, ARFF etc - if in attendance.