Speaking to the Advocate with her co-worker Te Hiwi Preston, Ms Lyndon-Tonga said many young Northlanders struggled with money.
Being paid at a lower rate for extended periods could be disheartening, they said.
Youth workers would be more susceptible to pokie machines or other forms of gambling to supplement their low income, she said.
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."
Ms Gay said young people were "already extremely appealing" to many employers who paid proper wages to all workers, regardless of age.
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, 1600 were from Northland.