"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 stopped work at 9.30am and met 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.
Foodstuffs spokeswoman Antoinette Shallue said the union's claim it had been bargaining with the Royal Oak supermarket since September was misleading.
She said First Union served notice then, but a meeting date could not be agreed on.
The union negotiator then became unwell and discussions were but on hold until March, when the union lodged its claim. Both parties then met on April 3.
Ms Shallue said staff at the Royal Oak store had been given a pay rise in June last year, and the planned annual salary review was due to take place this June.
"The youth wage has only just come into effect today. Foodstuffs has said that we are open to all new initiatives which encourage employment opportunities.
"At this early stage, individual stores haven't made a decision about the adoption of the youth wage."
The union action has been criticised by Business NZ chief executive Phil O'Reilly, who said picketing against a lawful activity was not what unions should be doing.
"Companies that contribute strongly to their communities, offering employment and career opportunities to young people while investing heavily in their training, deserve better than this.
"Offering a first step on the employment ladder to a young person is one of the most valuable things a New Zealand company can do," Mr O'Reilly said.
The new youth wage 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.
Auckland Action Against Poverty spokeswoman Sarah Thompson said paying young workers $11 an hour would not help create jobs.
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.
