Fraud and wilful suppression trigger notice and penalty requirements, with time-limited assessment and settlement 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 shall serve a show-cause notice requiring payment of the specified tax with interest under section 50 and a penalty equivalent to the tax; pre-payment by the taxpayer may forestall notice, shortfalls may be pursued, and orders determining tax, interest and penalty must be issued within five years from the annual return due date or from the date of erroneous refund.