CAR 5.104 and CAO 40.1.7 apply ... You can read them on-line here:
HTTP://www.casa.gov.au/scripts/nc.dl...D::pc=PC_90902
As Whiskey says, you'll need a CASA CPL first, then the Grade 3 initial issue test.
"Approved Person" is defined in the CARs as a person with a very specific legal piece of paper issued by the Director of CASA. (my paraphrase)