Retailers "across the board' are denying workers extra hours, according to a union.
First Union general secretary Robert Reid said the practice was widespread among retailers who used it to control workers.
Unite Union is taking Burger King and McDonald's to court arguing they are cheating their staff of their rights to guaranteed hours and shifts.
But Mr Reid said they were not the only ones failing to provide sufficient guaranteed hours.
"What we would call underemployment or low contracted hours is endemic in retail throughout New Zealand. And it would probably be a disservice to name any company in particular. It's right across the board.
"It's another way that workers' can be disciplined, their work life can be controlled to make them more compliant."
About 300,000 people work in retail, and Mr Reid estimates some 70 to 80 percent of them work less than 35 hours a week.
But retailers reject Mr Reid's accusations.
Greg Harford of industry lobby group Retail NZ said most employers' did the right thing by their staff and were happy to give employees more hours, if those hours are available.
"It's often a question of matching supply and demand so retail businesses, like many businesses, have peak hours. They'll be looking to have people in there during those times when customers want to come in and shop.
"In most cases they'll be looking to existing staff to fill those hours in the first place because it's more efficient and easier to use people you've already got trained up."
Mr Reid said more hours and security of hours are usually top of mind for retail workers during pay talks with employers.
"With one major supermarket chain for example, we were able to get contracted hours, which used to have a minimum of seven [hours] or even lower, up to 20 hours a week."
Mr Reid said unions would be monitoring employers to make sure they offered hours that workers could live on, and reminding them of their obligations under the new legislation banning so-called zero-hour contracts.