Determination of unpaid tax: show cause notices require payment with interest and may attract penalty unless timely voluntary payment. 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 show cause notice requiring payment of the specified tax with interest under section 50 and a penalty. A taxpayer may pay tax and interest voluntarily before notice to avoid proceedings; payment within thirty days of a show cause notice prevents penalty. The officer shall determine tax, interest and a penalty and issue an order within three years from the due date for the annual return or from the date of erroneous refund.