Fair Work Act, Section 410, Para (1) (b) (ii), pages 345-346.
You don't have to be an AIPA (or union) member. You have to be "an employee covered by the agreement". If you are covered by the Long Haul Award, authorised PIA is valid whether you're an AIPA member or not. BUT, you must comply with the notification requirements in Section 414. If you're an AIPA member, they do this on your behalf. If you're not, you'd have to do it yourself.
So yeah, tread carefully. But it can be done. Just sayin', ya know......
