No we are not only for Americans, we have members from all over the globe, many of them are Australian. You can search for your Australian ancestors, but they might not be here.
We are a member contributed tree, one person - one profile, which means that not everyone has been added yet (we're probably a long way from everyone). When you add your ancestors, they will be here, and maybe along the way you will find out that some of them are already here.