Your domain password (EPP) would have been sent to you when you first purchased your domain.
However sometimes if you purchased the domain and the company listed their own details for contact then they would have received your password.
To retrieve your password go to www.ausregistry.com.au
Then choose "Tools" then "Recover Password".
Alternative you can click on this url to go there directly.
This needs to be done for each .com.au, .net.au and .org.au domain.