Assessment of non-filers: officer may assess to best judgment; valid return within thirty days withdraws order but liabilities remain. Section 62 authorises the proper officer to assess a registered person who fails to furnish a return after notice by assessing tax to the best of his judgement using all relevant material and to issue an assessment order within five years from the date specified for filing the annual return for the relevant financial year. If the registered person furnishes a valid return within thirty days of service of that assessment order, the order is deemed withdrawn, but interest and late fee obligations continue to apply.