(7A)An aircraft that carries persons on a flight, otherwise than in accordance with a fixed schedule between terminals, is employed in a private operation if: (a) public notice of the flight has not been given by any form of public advertisement or announcement; and
(b) the number of persons on the flight, including the operating crew, does not exceed 6; and
(c) no payment is made for the services of the operating crew; and
(d) the persons on the flight, including the operating crew, share equally in the costs of the flight; and
(e) no payment is required for a person on the flight other than a payment under paragraph (d).
Looking further into it, part a above puts the kybosh on advertising for
'friends' on a website or any public announcement. Still enough grey though for plenty of shonky charter operations in Australia.