Fraud and wilful misstatement: authorities may assess unpaid tax with interest and escalating penalty payment options. Where tax is unpaid, short paid, erroneously refunded or input tax credit wrongly availed by reason of fraud, wilful misstatement or suppression, the proper officer must serve a show-cause notice requiring payment of tax, interest under section 50 and a penalty equivalent to the tax, with the notice issued at least six months before the assessment time limit and supplementary statements treated as notices for matching non-fraud grounds.