Determination of unpaid tax: notice, voluntary payment options, interest and penalty consequences under GST rules. Where it appears that tax has not been paid, short paid, erroneously refunded or input tax credit wrongly availed or utilised for reasons other than fraud or wilful suppression, the proper officer shall serve a show cause notice specifying tax, interest under the Act and any penalty; a statement extending identical grounds to other periods is deemed notice. The taxable person may pay tax and interest voluntarily to avert notice; payment within thirty days of a notice precludes penalty. The officer, after considering representations, will determine tax, interest and a prescribed penalty and must issue the order within the statutory limitation period from the relevant annual return due date or the date of erroneous refund.