Determination of tax liability requires a show cause notice; timely payment with interest avoids penalty and further proceedings. Where the proper officer finds tax unpaid, short paid, erroneously refunded or input tax credit wrongly availed for reasons other than fraud or wilful misstatement, he shall serve a show cause notice requiring payment of tax with interest and penalty; the officer may serve a supplemental statement for additional periods on the same grounds. A taxpayer who pays tax and interest before notice may inform the officer and avoid further notice for that amount; payment within thirty days of a show cause notice prevents penalty. The officer, after considering representations, shall determine tax, interest and penalty and issue an order within the statutory assessment time limit.