You can find your registrar via the WHOIS service and looking at the information in the 'Registrar Name' field.
This will show the details for names in the following namespaces:
The name displayed in this field is what is referred to as the 'registrar of record' - which may be different to the company you registered your name with if you used a reseller.
For gov.au domain names, the registrar of record is the Digital Transformation Agency.
For edu.au domain names, the registrar of record is Education Services Australia.
Category: Managing your .au domain name