FYI I wrote to CASA - actually asking about the Piper PA32 Cherokee 6. Don't propose to put it verbatim here, but in short the guidance suggested
1. PA32 configured as 6 seater can be used to cost-share. PA32 as a 7 seater cannot
2. Provided no hire/reward (I said no money changing hands), a PPL can fly a 7 seater PA32 and bring their family/friends for a joyride around town as it is a private operation
3. Confirm "no such restriction exists on the maximum number of passengers that can be carried by a PPL holder who is qualified to conduct the flight and who meets the relevant recent experience requirements"