Ref: http://www.auf.asn.au RA Aus Operations Manual
Section 2.01.1
No person shall act as Pilot -in-command of a recreational aircraft operated under the provisions of CAOs 95.10, 96.32 (RA-Aus Registration) or 95.55 or any relevant legislation as amended from time to time, unless that person:
(a) is the holder of a valid RA Aus Student Pilot or higher level Certificate, as listed in section 2.05, issued or recognised by the RA Aus.
..and it goes on, but that's the part we're talking about.
So I guess the operative word is "recognised"
Haven't had time to check whether a PPL, with suitable endorsement for the aircraft, is recognised by RA Aus