The police have no place blocking workers from getting organised for better wages, the Council of Trade Unions (CTU) says.
CTU secretary Sam Huggard said two union officials were served with trespass notices when they tried to enter the Nelson New World and Richmond Pak'nSave supermarkets while community rallies were taking place outside during the weekend.
Mr Huggard said employers should not rely on the Trespass Act to prevent workers getting access to union representation.
"Almost all employers in New Zealand understand and recognise the role of unions and have a reasonably cordial and straightforward relationship.
"It's really only employers at the outlines and some of the supermarket chains. I suppose our message to them would be to focus on talking to union and workers about lifting their low wages."
The union believed the owner-operator stores paid the Nelson workers up to $2 an hour less than staff in other centres, where supermarkets had a national agreement, Mr Huggard said.
The owners of two supermarkets said they had strong concerns about the union's approach.
New World Nelson City owner Greg Guy said the store was engaged in a good faith bargaining process with First Union, and he believed the union's approach at the weekend amounted to a breach of good faith.
Pak'nSave owner Andrew Howard said they were also engaged in a bargaining process with the union.