A crowd gathered outside Burger King's upper Queen Street store on Sunday to protest against alleged unfair treatment of migrant workers by the fast-food company.
About 50 unionised workers, supporters and representatives from Unite Union turned out for the protest from about midday in central Auckland.
Mike Treen, national director of Unite, said Burger King management told workers there would be serious consequences for those who attended the protest.
"Anyone who joins the union is subject to intimidation and pressure to leave," he said.
"If they go to the union to get help they face even more victimisation."
The union has been flooded with messages from workers "telling horror stories of how they've been treated".
Further protests are planned over the coming weeks.
Unite said earlier this week that Burger King was discriminating against migrant workers, especially those from India.
"To get an offer of permanent residence they need to be offered a manager's job," Unite said.
"This results in many of the migrant workers agreeing to do anything and everything demanded by the BK bosses."
Burger King representatives could not be reached on Sunday.