Many of the Australian languages have already died out. Although their respective grammars exhibit a great degree of variation, the Australian languages still show many similarities. All of them inflect the noun, some having as many as nine cases. The verb lacks a passive voice. Postpositions are used instead of the prepositions typical of Indo-European languages.
Word order tends to follow a similar pattern in the different tongues.
They also show considerable similarity phonetically and have a small common vocabulary. Because of so many shared phonetic and grammatical characteristics some scholars believe that the Australian languages have all evolved from a single ancestor language and therefore belong to the same linguistic family. The Columbia Encyclopedia, Sixth Edition.