These sites do a lot to explain about the arrival of the Germans in Australia.

German Australia - German Speakers in Australia
(Click on Issues, Reasons for Emigrating, What effects did WWI have on German speakers in Australia...)

German Settlers in South Australia

Immigration Museum - Immigration to Victoria 1850s Australia

General reading...The Nieder-Weisel Story

And if you want to research how your forbears arrived in Australia,-

National Archives of Australia
(click on 'record search', 'search now as a guest'...)

If you have some joy in finding your ancestors, look at the relevant State Records Offices of the various states in Australia, particularly New South Wales and Victoria. There may be passenger arrival details, etc.