Carter Holt Harvey has confirmed its Eves Valley sawmill in Tasman will close with the loss of more than 140 jobs.
Tasman Mayor Tim King and Nelson Mayor Nick Smith had appealed to the company to delay the closure for a year so the "wall of wood" left behind from winter storms could be salvaged.
But E tū said the company has now confirmed it would close its operations that have been running since the 1980s.
Union delegate Maria Hemara says she and her colleagues are "broken" after learning they have lost their jobs.
Hemara has worked at the mill for six years and told Checkpoint workers had been preparing for the worst.
The closure will have huge flow on effects for the Tasman region, she said.
Hemara said she was devastated.
"I feel like I've lost my whole family. We work together for 40 hours a week, we've built friendships, and it's all being taken away. It's like going to your own funeral," she said.
"I'll be looking for jobs - I've tried supermarkets, and other mills around here. If not, I'll have to go temping [or] something.
"I think it will be a burden for the whole community. They will share our grief. The loss of jobs and productivity in the region isn't good for anyone."
The closure would be staggered from the start of next month until the end of November, E tū said.
RNZ understood Carter Holt Harvey wanted to consolidate resources in Kawerau in Bay of Plenty.
E tū national secretary Rachel Mackintosh said it was a strategic decision that ignored the human cost as the company was not going broke and instead chose to centralise its operations.
"But we're talking about people's lives here, and it's cold comfort for more than 140 workers who are now facing unemployment in a region already hit hard," she said.
"These workers, many of whom have put decades of their lives into the mill, are now caught in the crossfire of a corporate decision."
E tū was calling on Carter Holt Harvey to support its staff and said the government needed to step up.
"Instead of shrugging its shoulders at rising unemployment, the government must be part of the solution, with targeted support for affected workers and communities."
