Assessment of unregistered persons permits best judgment tax assessment with prior hearing and a statutory time limit for orders. Section 63 allows a proper officer to assess the tax liability of a person who was liable to obtain registration but failed to do so, or whose registration was cancelled yet remained liable to pay tax, by making a best judgment assessment for the relevant tax periods and issuing an assessment order within the time limit tied to the annual return for the relevant financial year; the person must be given an opportunity of being heard.