I think in this case you lost your German citizenship since your mother and you took the Australian citizenship.

I think the easiest for you is to go the UK since they have these special visas for Australians. In case you want to go to Germany you can apply for a resident permit and work permit after you entered Germany. Still I recommend to take a look on the website of the Germany Embassy in Australia or top contact them. To get resident permit takes a lot of paper work and you have to show a lot of documents to the authorities. So better to get it done before hand.