I think it depend on whether the university choose to provide a language course or not. In case of my university, UniSA, they do have a language course to prepare international students to study at UniSA. The duration of the course depend on your level of enlish. In all, it is important that you know the location you will be in Australia and find out more about some respectable Uni/colleges in the area and whether they offer a language course.

BTW what is "sejour linguistique en Australie" =p