The union said more than half of its 870 members - around 30 per cent of Farmers' workforce, were on or below this grade.
First Union retail, finance and commerce secretary Tali Williams said Farmers workers want the retailer, owned by the James Pascoe Group, to phase out the system that holds wages down.
"We realise this cannot happen overnight, but for ethics' sake it should be removed. How can they tout their brand as a family's brand when at the same time their abysmal pay is causing many families to struggle?," Williams said.
Farmers, which markets itself as the family neighbour in its Christmas TV advertisements, has been approached for comment.
First Retail Group managing director Chris Wilkinson said Farmers needed to review its system of pay rates to ensure they were "contemporary".
"In this situation with Farmers, it may require a more contemporary review of their system," Wilkinson said.
"Reward must relate to performance and one of the biggest challenges that retail has at the moment is finding quality staff. Reward needs to reflect the efforts, the skills and the accounts that people are bringing to these roles," he said.
"Big employers need to have these types of structures, particularly when they have large workforces dispersed over a large area."
Wilkinson said Farmers and James Pascoe Group's other retail stores were competing in an "incredibly" challenged market and categories.
"Farmers has, by contrast [to Australian counterparts such as Myer], managed to retain staff, grown stores, and we haven't seen those big redundancies that have happened in other parts of Australasia in those categories," he said.
"The heads of Farmers have been acknowledged over the last decade for just how they have handled these very challenging sectors they're in."
Williams said Farmers pay scale was substantially behind other retail employers.
"It's disappointing to see Farmers, a company steeped in so much New Zealand history as an iconic Kiwi brand, lagging so far behind in a time in New Zealand's history when businesses are beginning to acknowledge they need to pay wages that workers can thrive on, not just survive," she said.
But Wilkinson said he was unsure if that was the case.
"There are a lot of retail groups reviewing their systems, mainly because employment is causing challenges. Securing the right kind of people is really important, being able to develop capabilities and these systems at times do have some very strong training systems."
He said Farmers had given "tens of thousands of New Zealanders" their first job.
James Pascoe Group owns Farmers, Whitcoulls, Pascoes, Stewart Dawsons, Stevens and Goldmark.