Employment Relations Act 2000, s 52(3)
A collective employment agreement comes to an end on the expiry date stated in the agreement, or when the event specified in the agreement happens, or three years after the agreement begins, whichever of those three dates or events comes first.
But if the union or the employer initiates bargaining before the agreement expires, it continues in force for up to 12 months or until it’s replaced within the 12-month period with a new collective agreement.