I think that hamsters are seen as a potential danger to Australia's very small marsupials such as the kowari. Though they wouldn't be competing for food (kowaris are little carnivores) they certainly would compete for burrows.
Edited to add - I read that hamsters in the wild eat insects so there would be competition for food.
Edited to add - I read that hamsters in the wild eat insects so there would be competition for food.