Determination of tax not paid triggers show cause notice requiring payment with interest and possible penalty under tax recovery provisions. Where a proper officer finds tax unpaid, short paid, erroneously refunded or input tax credit wrongly utilised (other than due to fraud or wilful suppression), he shall serve a show cause notice requiring payment of the tax, interest and applicable penalty; a detailed statement may extend the same grounds to other periods. Voluntary payment of tax and interest before notice avoids further notice for that amount; payment within thirty days of notice prevents penalty. The officer shall determine tax, interest and penalty after representation and issue an order within the statutory limitation period.