Supermarket company Foodstuffs stands accused of threatening to sack temporary staff who are considering joining a union.
However, Foodstuffs say the union claim is baseless.
A group of 70 permanent workers at CTD Nesdale, a south Auckland chilled and frozen food centre, have walked off the job in protest at the company's treatment of temporary workers.
The factory is one of four wholesale distribution centres for the upper North Island supply chain.
Workers and supporters are picketing outside the distribution centre in Wiri, near Auckland Airport, in a planned week-long strike.
First Union said temp workers at a meeting yesterday were threatened with the sack if they spoke to the union about joining up.
Union organiser Lisa Meto Fox said that was illegal and unethical.
"This is a perfect example of why we are taking this industrial action because these temps are in an incredibly vulnerable position.
"They have no job security which means they can be threatened with things that are illegal like 'don't join the union or we'll sack you."
The Foodstuffs group includes the New World and Pak'n'Save supermarket chains.
In a statement, Foodstuffs North Island chief executive Chris Quin said the union's claims were baseless.
"We are astounded to read the claim by the union that seven people were fired for taking part in the strike", he said.
"There is no truth to [this allegation]. The mood on site has been quite amicable."