What is the cheapest way to send a bike from Australia to Germany?
Check with the airline you are flying with. Airlines generally have a small surcharge for taking a bike along, since people frequently travel with sporting equipment.

I looked into flying with a bike from Canada to France last year and it was only a $30 charge on the airline I was flying with (they even accepted tandems). I can't guarantee your airline will be as reasonable, but it's definitely worth looking into.