Determination of unpaid tax requires notice, interest and potential penalty, with limited time for assessment and payment. Where a proper officer finds tax unpaid, short paid, erroneously refunded, or input tax credit wrongly availed or utilised (other than by fraud or wilful suppression), he shall serve a show cause notice requiring payment of the specified tax with interest and a penalty. The officer may issue supplemental statements for additional periods on the same grounds. Voluntary payment of tax and interest before notice avoids issuance of notice for that amount; payment within thirty days of a show cause notice precludes penalty. The officer determines tax, interest and a prescribed penalty and must issue an order within the statutory limitation period.