Labour leader Andrew Little has conceded his 'Ready for Work' scheme is a six-month policy that has been costed for four months.
Talking to Paul Henry, Mr Little said the programme would "cost a little more" if participants stayed on it for longer than the international average of four months.
The 'Ready for Work' scheme was released at the Labour Party's Annual Conference over the weekend. Under the scheme anyone under 24 years of age who's been on the Jobseeker's Allowance for six months will be offered a job by the Government.
Andrew Little expects 10,000 youths to sign up every year, around one sixth of young people who are not studying or working.
Mr Little says claims Labour has under-costed the policy is "National Party spin". He says the costing is based on the average time spent in similar programmes, which is four months.
"We've taken the information we know about these programmes from here and overseas based on evidence.
"We've looked at the cost now and we've taken into account all costs, the extra tax that would come from it as well. The figure of $60 million per year is accurate, I stand by it, there's nothing wrong with it."
However if young people in New Zealand do not follow the trend then there could be a problem.
"In the unlikely event that that would happen - because it would be completely contrary to evidence of the way these programmes work - it'll cost a little more."
But Prime Minister John Key says Labour needs to do more work on the plan: "They just can't get their numbers right, it's not that hard, this is a pretty simple thing."
He says the party needs to change its attitude or it won't see any improvement in the polls.
"They are lazy. Go and have a look at the questions they are asking in Parliament, a lot of the time they are not terribly well-researched.
"They are not drilling down on a particular issue that often. It's pretty superficial numbers that you can shoot down."
But Mr Little says it's the Government that's not working hard enough to safeguard the future of the country.
"We've got a major problem; young unemployment is going up under this Government. They've got no sense of anything to do about it and we do and that's what this programme is about.
"It is about giving young people a chance and giving them hope."