Determination of unpaid GST: notice, interest and penalty framework for non-payment and wrongful input tax credit claims. Procedure requires the proper officer to serve a show cause notice for tax not paid, short paid, erroneously refunded or wrongly availed input tax credit (other than fraud), specifying tax, interest under section 50 and penalty; officer may serve additional statements deemed as notice if grounds match. Voluntary payment of tax with interest before notice precludes proceedings for that amount, but shortfall may be pursued. Payment within thirty days of notice waives penalty. Officer determines tax, interest and penalty after representations and must issue the assessment order within the statutory limitation period; penalty applies where self-assessed or collected tax is unpaid beyond thirty days of the due date.