Determination of unpaid tax triggers notice, interest and potential penalty with limited time to assess and remedy the default. Where the proper officer finds tax unpaid, short paid, erroneously refunded, or input tax credit wrongly availed for reasons other than fraud or wilful suppression, he shall serve a notice requiring show cause and payment of tax with interest and a penalty. The person may pay voluntarily before or after notice; timely payment can preclude penalty and further notice for that amount. The officer, after considering representations, will determine tax, interest and penalty and issue an order within the statutory limitation.