It is not hard to call yourself an auditor. A good eye for detail and a knowledge of the AQTF standards and how they relate to an RTO is a good starting point.

Auditors have benchmarks. Appropriate qualifications and experience lead to auditors becoming registered. This is not a simple 'membership' but a system which requires evidence of experience and knowledge in the field of quality management systems auditing to gain registration.

Whilst there is a number of auditor certifying bodies, the most prominent one in Australia is the Registration Accreditation Body - Quality Society of Australasia.

If you want to see if your auditor is registered you can check this public register.

http://www.rabqsa.com/internalqsa/?module=audSearch&func=search&schemeID=1