Assessment of unregistered persons permits officer to estimate unpaid tax where liable persons failed to register, with right to hearing. Where a person liable to pay tax fails to obtain registration or has had registration cancelled but remains liable, the proper officer may assess that person's tax liability to the best of his judgement for the relevant tax periods and issue an assessment order within a five-year period measured from the date specified for furnishing the annual return for the financial year to which the unpaid tax relates, provided the person is given an opportunity of being heard.