The National Distribution Union says Fletcher Building has confirmed it will close a factory north-west of Auckland, resulting in the loss of 41 jobs.
The plant at Kumeu produces particleboard for homes.
Workers at the mill were warned about the possibility of the closure earlier in June.
National Distribution Union president Robert Reid says the closure is major blow to the workers and the local community.
The union says the Fletcher Building made the announcement at a meeting on Friday morning.