In Austria, subletting is generally permitted under the Austrian Civil Code (ABGB Allgemeines bürgerliches Gesetzbuch).
The landlord may only invoke a contractual prohibition against the sublease, however, if there is good cause.
Good cause is said to exist if:
The phrase "completely rented" is somewhat unclear. According to Art. 30 of the Law on Tenancy, the phrase refers to time, not space: "In particular, good cause exists if the lessor has fully rented the rental property ... and clearly will not at any point in the near future ...require it for him/herself."
So if you sublet your entire flat for a certain period of time and there is a clear intent on your part to occupy the flat once more yourself after this period, you are not renting it completely.
No responsibility is taken for the correctness of this information.