Payment in instalments: Commissioner may allow deferred GST payment subject to interest; default accelerates the entire outstanding balance. Section 80 allows the Commissioner, for reasons recorded in writing, to extend time or permit payment of amounts due under the Act by monthly instalments subject to payment of interest under section 50 and prescribed conditions, excluding liabilities self-assessed in returns; default of any instalment causes the whole outstanding balance to become immediately due and liable for recovery without further notice.