Assessment of non-filers: officer may assess tax to best judgment; valid return withdraws assessment but interest remains. Where a registered person fails to furnish a return after notice, the proper officer may proceed to a best judgment assessment of tax liability using available material and issue an assessment order within five years from the date specified for filing the annual return for that financial year. If the person furnishes a valid return within thirty days of service of that assessment order, the assessment is deemed withdrawn but liabilities for interest and late fee continue.