Southland's Tiwai aluminium smelter faces accusations of a ban on free speech, as it cracks down on emails sent by its workers.
Union E Tu says New Zealand Aluminium Smelters (NZAS) has a "gagging clause in its employment agreements".
An internal memo has banned employees from forwarding company messages, sharing information with outside parties via email and storing company information on external services such as Dropbox or iCloud, according to an e-mail sent to staff.
The email said to "keep information inside the gate", meaning never supplying "any NZAS information to a person outside our business without gaining approval".
E Tu organiser Trevor Hobbs believes it's an underhanded attempt to choke off communication.
"I think it's just another attempt by the employer against the union to prevent it getting itself firmly established on the worksite at Tiwai Point.
"This employer is really, really active in working against the union. I think it's a breach of human rights."
Last month, NZAS was denied the right to take an employment case heard by the Court of Appeal to the Supreme Court, in which workers asked for an extra day's leave for each public holiday worked.
The company said this should not apply if the public holiday fell on a day when the employee would not be working, but then workers said they should get an extra day regardless.
NZAS were not available for immediate comment.