Foodstuffs has been accused of firing temporary workers at a south Auckland distribution centre if they talk to their union.
First Union spokeswoman Lisa Meto Fox said the distribution manager called a meeting with workers on the first day of the strike on Thursday and seven workers were fired for speaking to the union.
But Foodstuffs' chief executive Chris Quin told Newshub they were "astounded" to read the claim.
"This is a baseless allegation. There is no truth to it," he said.
"The mood on site has been quite amicable."
In the statement, spokesperson for FIRST Union Lisa Meto Fox said the move was unlawful and condemned the company.
"This is typical bullying behaviour from a company that exploits the labour-hire workers' insecure work arrangements. This is a highly profitable company preying on some of the most vulnerable workers in society."
The union is laying a complaint to the Recruitment and Consulting Services Association (RCSA), the body representing labour-hire firms, and is looking to take the matter to the Employment Relations Authority.
"We will defend every worker who is dismissed for exercising their legal right to freedom of association," said Ms Meto Fox.
Around 70 Foodstuffs employees at the chilled and frozen food distribution centre are on strike over the company's treatment of hundreds of labour-hire employees in place of permanent employees.
The strikers are calling on Foodstuffs, which owns Pak'nSave and New World supermarkets, to do more to ensure temporary workers receive decent employment conditions.
On Thursday, Mr Quin said they believed the offer put to the union was fair and reasonable.
The workers had made bargaining claims and given letters from temporary workers which described the toll their work and the lack of employment protection had on their lives.
Ms Meto Fox said most temporary workers were still working with no guarantee of hours and very few protections under employment law.
Newshub / NZN