First Union, which represents about 12,000 workers in the retail sector, called the bill "shameful".
"It's utterly appalling and is really yet another attack on workers," retail secretary Maxine Gay said.
Kmart, Countdown, Bunnings and Farmers were some of the major chains which operated without a youth wage rate, she said.
"They all operate completely and utterly successful without youth rates."
Proponents who argued the changes would reduce youth unemployment were wrong, she said.
About 41,500 Kiwis aged 15 to 19, who are registered as part of the Labour Force, were unemployed in the three months to December, Statistics New Zealand estimates showed.
Of these 1900 were from the Bay of Plenty region. Specific figures for Rotorua were unavailable.
The Human Rights Commission has also weighed in on the debate.
"In New Zealand, the age at which children and young people are deemed to be adult is considerably younger than 20 in many critical areas of life," the organisation's submission said.
Children were able to marry, with parental consent, and drive at 16. Only a year later, they could enlist in the military and be deployed at 18, it said.
"The minimum age of prosecution is 14 for most offences, 12 for serious offences and 10 for murder and manslaughter."
"Yet they are not considered to be sufficiently adult enough to be protected by the minimum wage."
National, Act and United Future support the bill, saying it would increase the likelihood of youngsters being taken on by employers, who often prefer experienced workers.