If you are allocated an exchange place, you will automatically be nominated to the host university. You will then have to complete an application to your host university. You will be informed about the application to the host university by email from AU or directly from your host university.
Please note that being recommended for an exchange place is no guarantee of admission to your host university. During the process of applying to the host university, a situation may arise which AU was not able to foresee when allocating the exchange place. At the end of the day, the host university decides whether to admit you or not.