Union organiser Darien Fenton said workers were on Friday given a letter saying a refusal to work on the Waitangi Day public holiday could amount to an unlawful strike and would jeopardise their employment.
She said a further letter sent by AFFCO to the union yesterday reinforced that threat.
The meat company and the union disagree over their interpretation of the Holidays Act, but Ms Fenton said the union would support its members no matter what they decided.
"They're asserting their right to have a day off today - which is what the whole Waitangi Day Mondayisation thing is about."
She said some staff were excercising that by not going to work today.
"Our advice to members and our advice to the company is that they're entitled to assert their rights to the day off because their agreement doesn't provide that they can be required to work."
The company's letter said today was an ordinary working day for those who were required, and staff required to work had been advised.
"A failure to attend work on that day may give rise to disciplinary action, and could be considered an illegal strike."
It said AFFCO was operating its business within the requirements of the law.