Credit and debit notes: adjust GST liability when invoices are amended; suppliers must report amendments in returns within prescribed timelines. A supplier may issue a credit note where a tax invoice overstates taxable value or tax, goods are returned, or supplies are deficient, and must declare such credit notes in the return for the month of issue within prescribed time limits; no output tax reduction is permitted if tax and interest have been passed to another person. Conversely, a supplier must issue a debit note where a tax invoice understates taxable value or tax, declare it in the return for the month of issue, and adjust tax liability as prescribed. 'Debit note' includes a supplementary invoice.