Determination of unpaid tax: notice-led assessment requires payment with interest and possible penalty. Where a proper officer finds tax unpaid, short paid, erroneously refunded, or input tax credit wrongly availed for reasons other than fraud or wilful misstatement, the officer must serve a notice requiring the person to show cause and pay specified tax, interest and penalty. A statement may extend the periods covered if based on the same grounds. Voluntary payment of tax with interest before notice precludes proceedings for that amount; shortfalls may be pursued. Payment within a short period after notice removes penalty liability. The officer must determine and order tax, interest and penalty within the statutory assessment period.