Determination of tax not paid triggers show cause notice, interest, and prescribed penalty unless tax is voluntarily paid promptly. The proper officer may issue a show cause notice for tax not paid, short paid, erroneously refunded, or input tax credit wrongly availed or utilised (excluding fraud or wilful suppression), requiring payment of tax with interest under section 50 and applicable penalty. A supplementary statement for additional periods on the same grounds is deemed a notice. Voluntary payment with interest before notice precludes issuance for that amount; payment within the cure period after notice waives penalty. The officer shall determine tax, interest and penalty and issue an order within the statutory time bar measured from the relevant annual return due date or erroneous refund.