Your GST return and payment must be filed by the last working day of the month after your return period finishes. (For returns due by the last day of December, you have until 15 January because of the Christmas break.)

Make sure you send in your return and payment on time - The IRD imposes a penalty of 10% plus a further penalty of 2% each month thereafter and in addition can hit you with a late filing fine. Other penalties apply to other taxes such as provisional tax so check those out separately.