Well considering many were born in Sudan or were born here to parents who were born when the country was Sudan or migrated when the country was still Sudan, I think it is equally applicable to call them Sudanese as that is what they were when they were born or that is what their parents were when they migrated to Australia.
Just another little fact, many of the South Sudanese here say they are Sudanese for short but seeming you don't live in Melbourne I guess you probably wouldn't know that.