Determination of unpaid GST triggers notice, interest and penalty unless tax paid within thirty days. Where tax is not paid, short paid, erroneously refunded or input tax credit wrongly availed or utilised other than by fraud or wilful suppression, the proper officer shall serve a show cause notice requiring payment of the tax, interest under Section 50 and penalty. The officer must issue the notice at least three months before the outer time limit and may serve a deemed notice by statement for additional periods with the same grounds. Voluntary payment of tax and interest before notice bars subsequent notice for those amounts unless short, and payment within thirty days of notice waives penalty. The officer shall determine tax, interest and a penalty and issue an order within three years from the relevant annual return due date or from the date of erroneous refund.