A Canterbury meat plant which is part of the Talley's group has been fined $28,000 after it kicked out and tried to trespass a union rep trying to recruit new members.
The Meat Workers Union says it is the largest Employment Relations Authority penalty for its kind of case.
The union took the case against South Pacific Meats after organiser Wayne Ruscoe made an agreed-to visit to the Malvern plant in May this year.
The ERA decision says Mr Ruscoe was handing out sealed envelopes to workers but refused to show what was in them to health and safety officer James McFarlane.
It led to a quarrel, caught on CCTV, over whether Mr Ruscoe could film and where he could go in the factory before a "burly" worker grabbed Mr Ruscoe and pushed him against a corridor wall.
Mr McFarlane later served him with a trespass notice, accusing him of disorderly behaviour, assaulting him with a door and unlawful filming. He also said police had suggested the trespass notice.
But the ERA says Mr Ruscoe had a statutory right to be on the premises and it considered Mr McFarlane obstructed him.
ERA member Christine Hickey said police had only suggested a trespass notice if Mr Ruscoe tried to return, and it was an illegitimate attempt to deny future access.
SPM should have known it couldn't trespass unions and she issued the company with a compliance order to allow Mr Ruscoe to visit.
She also said Mr Ruscoe had been assaulted by the worker, although she accepted he hadn't been instructed to manhandle him.
SPM has a counter claim with the ERA, alleging the union is acting illegally and Mr Ruscoe was distributing false, misleading and derogatory comments.
The union says SPM has been fined over $150,000 in recent years for more than 20 earlier union access breaches at their meat processing plants in Canterbury and Southland.
"No employer is above the law and unions have a statutory right to visit work sites. We will keep doing our job of working for worker's rights at SPM and in the wider Talley's group," said national secretary Graham Cooke.