If you live in Australia, possess Australian (or another) citizenship in addition to German citizenship, and, for certain reasons, wish to renounce your German citizenship, you must file an application through the Consulate General in Sydney.

Further information as well as the application form (in German) can be found here

If you wish to renounce your German citizenship, please get in touch with the Consulate General in Sydney via info@sydney.diplo.de .
Your application will be forwarded by the German foreign mission to the German Federal Office of Administration in Cologne, Germany. If renunciation is approved, a renunciation certificate will be issued to you. It will be handed over to you by the foreign mission. The renunciation does not enter into force until you accept the certificate.