The Meatworkers Union says a dispute at an AFFCO plant at Wairoa will be taken to the Employment Authority.
More than 200 employees on the last lamb and mutton chain still operating for the season were laid off on Thursday.
The union says it's unclear how long they will be out of work.
It says the company told dayshift workers last week that their season would finish unless they were prepared to work harder for no more money - and that raised the prospect of no work for up to five months.
In an apparent reprieve, the union says, the company then indicated it would keep one chain going - but the situation changed again on Thursday.
Area organiser Eric Mischefksi complains that the company keeps shifting the goalposts about its intentions.
Mr Mischefski says a core agreement covering AFFCO meat works in the North Island was recently ratified and the Meatworkers Union expects the company to honour that agreement.
No choice, says company
AFFCO says staff have not been locked out.
Operations director Rowan Ogg says the company had no choice but to action an early closure for its sheep and lamb plant in Wairoa because it is not meeting production requirements.
Mr Ogg says the Wairoa plant is not its most productive operation.
Beef processing at the plant is not affected.