Determination of unpaid tax: show-cause notice, voluntary payment options and time-limited assessment consequences. Where a proper officer finds tax unpaid, short paid, erroneously refunded or input tax credit wrongly availed for reasons other than fraud or wilful misstatement, a show-cause notice specifying tax, interest under section 50 and liable penalty must be served; a statement extending the same grounds to additional periods may be deemed notice. Voluntary payment with interest before notice prevents issuance of notice for the paid amounts; payment within thirty days of notice precludes penalty. After representations, the officer determines tax, interest and penalty (ten per cent of tax or ten thousand rupees, whichever higher) and must issue the order within three years from the relevant annual return due date or erroneous refund date.