Determination of tax not paid: notice triggers interest and penalty unless tax is voluntarily paid within the prescribed period. Determination of tax not paid, short paid, erroneously refunded or input tax credit wrongly availed or utilised (other than by fraud or wilful misstatement or suppression) is commenced by a show cause notice requiring payment of the specified tax with interest under section 50 and a penalty. Voluntary payment before notice bars further proceedings for that amount; payment within thirty days of the show cause notice eliminates penalty. The proper officer must issue the final order fixing tax, interest and a prescribed penalty and do so within three years from the due date for the relevant annual return or from the date of the erroneous refund.