Which one?

The world of medicals can be confusing. We mainly deal with 3 types.

Class 1- Commercial pilots (must be done every 12 months)

Class 2- For PPL and RPL last 4 years if your under 40 and 2 if over 40)

Both these must be done through a DAME (Designated Aviation Medical Examiner).

http://pilotmedicalsaustralia.com.au will put you in touch with Dr John Lancashire who can take care of these for you.

Basic class 2- Has more limitations like a maximum of 5 passengers, no flight above 10,000ft, no night flight etc. Can be issued by a wide range of doctors but have less flexibility.

more info here https://www.casa.gov.au/licences-and-certificates/aviation-medicals-and-certificates/classes-medical-certificate

As you prepare for a PPL or RPL CASA exam you will also need a good understanding of the different medicals, as it is often an exam question.