The department said the rest of those young people had lost their jobs over that period because of deteriorating economic conditions.
O'Reilly said the loss of part-time jobs was not a surprise.
It was to be expected when there was higher unemployment. Employers were now more cautious about hiring school children and had extra health and safety concerns to consider.
Many of the traditional low-skill jobs are disappearing.
"The other reason is the lack of a youth rate. If you have to pay an adult rate, you may as well hire an adult." He said many young people did not have work-ready skills such as teamwork, customer-service skills and punctuality.
Council of Trade Unions (CTU) president Helen Kelly said young people were the "last cab off the rank" when it came to employment.
"If there is a shortage of jobs for young people, it's because there is a shortage of work," she said.
CTU spokesman Bill Rosenberg said there had been less youth employment since the abolition of youth rates but there were fewer "inactive" youth, which indicated that young people were choosing to study rather than trying to enter the workforce.
Progressive Enterprises general manager of human resources Gillian Davie said the workforce at its Countdown, Woolworths and Foodtown stores had become older.
She attributed that to an ageing workforce and people staying in work longer. "Year on year, we have seen the percentage of workers in the younger age group slowly decreasing."
Davie said age was not part of their hiring decision. "We hire the right person for the job."
McDonald's was approached but would not comment, saying the issue was too political.
Auckland Youth Advisory Panel chairman Sam Crosbie said teenagers having part-time jobs had been linked to a reduction in crime. Not having one could cause huge frustration. "You can't afford to go out or drive a car."
He said it was a vicious cycle that employers would choose people with experience but teenagers could not get that first experience.
O'Reilly said New Zealand had traditionally been an economy that had a high proportion of young workers.
"We tend to think if young people can help out in mum and dad's dairy or on their uncle's farm, that's good experience."
He said the lack of a job while at school could affect later-life career choices.
"If they are working after school for an hour or two, that gives them the capacity to make choices later in life. We are poor at informing young people in school about their career choices."
It was a concern that when the economy picked up there might be a "two-tier" effect where skilled, experienced people had no trouble getting jobs but those without basic skills were left as a stubborn group of unemployed. "Young people are competing with older workers."
Out of luck
Sean Chambers, 16, is in Year 11 at Tauraroa Area School in Northland. He has been applying for part-time jobs in Whangarei without success for the past year.
Pak'n Save had a huge waiting list - "there were pages of people applying" - and he approached Burger King but was told he would have to wait until the next recruitment intake because of the number of applicants.
He reckoned about a quarter of the students in his year had jobs but many were able to get work on the family farm.
