The change allows businesses to pay 16- to 19-year-old workers a minimum wage of $11 an hour for their first six months - which is 80 per cent of the $13.75 adult minimum wage.
Labour Minister Simon Bridges has said the youth rate would give many young people an opportunity to work which they otherwise would not have.
It has been rejected by large retailers including The Warehouse, Farmers, Kmart, Bunnings and Countdown, as well as fast food giant McDonald's and rival Restaurant Brands, which owns Pizza Hut, KFC, Starbucks and some Carl's Jr stores.
But Foodstuffs, which owns the Pak'n Save and New World brands, is pushing for the introduction of the youth wage in pay talks at some supermarkets.
Members of the First Union and Auckland Action Against Poverty are picketing outside the Pak'n Save in Royal Oak this morning to protest the move.
First Union retail secretary Maxine Gay said the union had been bargaining with the supermarket since September last year.
"In all these months they have offered no pay rise, and are trying their hardest to bring in youth rates," she said.
Ms Gay said Foodstuffs' businesses were extremely profitable before the youth rates came into effect today.
"They are simply chasing an opportunity to get away with paying young workers less in order to make even greater profits."
Workers at the Royal Oak supermarket will stop work at 9.30am and meet colleagues from five other Pak'n Save supermarkets and Foodstuffs' two Auckland distribution centres.
Ms Gay said the meeting was the first time workers from both the retail and distribution side of the business had come together.
"This reflects the growing anger among workers about the poverty wages the company pay," she said.
Auckland Action Against Poverty spokeswoman Sarah Thompson said paying young workers $11 an hour would not help create jobs.
"We know that as time goes by older workers will simply be displaced by younger ones at lower wages, shuffling people on and off the dole queue.
"It's also unacceptable that under the new law 18- and 19-year-olds may be forced on to youth rates if they have been on the benefit for six months or more."
Labour Party labour spokeswoman Darien Fenton said there was not a lot to celebrate today on May Day, which is International Workers' Day.
"At a time when we should be celebrating real progress for New Zealand workers, we are instead seeing wages cut for young workers and a Government determined to take workers' rights back to the 1990s."
The new youth wage can be paid to:
* 16- and 17-year-olds in their first six months of a new job.
* 18- and 19-year-olds who have been on a benefit for six months or more.
* 16- to 19-year-olds in a recognised industry training course.
