Determination of unpaid tax requires notice and allows payment with interest to avert penalty and assessment order. Section 73 sets the procedure for determining tax not paid, short paid, erroneously refunded, or input tax credit wrongly availed or utilised (other than by fraud or wilful suppression). The proper officer serves a show cause notice (or a statement for additional periods on identical grounds), permits preemptive payment of tax with interest by the taxpayer, and may issue further notice if payment is short. Timely payment after notice avoids penalty. The officer, after considering representations, determines tax, interest and the prescribed penalty and must issue an order within the statutory limitation tied to the annual return or erroneous refund.