Back to the core subject of this thread...
CASA's most recent response confirms that the rules do not impose a maximum seat configuration for a private operation:
Please find the response to your query below:
Scenario: I as PIC meet the entirety of the direct costs of a flight in an aircraft I own with a maximum seat configuration of 15 (King Air 350ER), with the corollary that no one else is meeting any of the direct costs of the flight. I receive nothing from anyone for anything connected with the flight. There is nothing in the type certificate or flight manual prohibiting the carriage of 14 passengers in the aircraft.
I note that CASA has agreed that that there is no “reward” if the PIC meets the entirety of the direct costs of a flight, with the corollary being that no one else is meeting any of the direct costs of the flight, and the PIC receives nothing from anyone for anything connected with the flight.
Question: May I lawfully conduct the scenario flight as PIC under the authority of my private licence alone, assuming I have the necessary ratings/endorsements for the aircraft?
Answer: Yes. The aviation legislation does not impose a maximum seat configuration limitation for a private operation.