National's decision to increase classroom sizes was made after last year's election, Education Minister Hekia Parata has confirmed.
The admission comes as Ms Parata has rejected a call to urgently meet education groups that have united in opposition to funding proposals that will increase class sizes.
Instead, she says, she will try to defuse their anger and meet the organisations individually this week.
Ms Parata confirmed to Radio New Zealand this morning the decision was made after National was re-elected for a second term last year.
Labour has argued National would not have been re-elected had the class sizes policy been known to the electorate prior to the election.
"I can't tell you the exact date [the policy was decided on]," Ms Parata told Morning Report, "but I can tell you over the last weeks and months we have been trying to get to grips with why - year on year - one out of five students is being left behind.
"We've been working on a range of issues which would allow us to focus further on raising student achievements. There was a set we focused on last term of Government when we were in, those included how we get better data and assessment for Years 1-8.
"We came back into government in November, which gave us the opportunity then to think about what we could do to focus on lifting the one out of five as well as improve the whole system."
An alliance of unions, principals organisations and the Schools Trustees Association met in Wellington yesterday to call for a reversal of the proposed student-teacher ratios and immediate discussions with the Government.
Ms Parata declined the offer of a meeting, saying last night that it was something she could consider down the line.
But she suggested that there was room for negotiation on provision of technology teachers for classes such as art, music, cooking and woodwork.
In an embarrassment for the Government, Ms Parata announced new ratios as part of the Budget without realising that among the 244 adversely affected schools, some could lose up to nine teachers.
Last week she hastily limited lost positions to no more than two full-time equivalents over three years. But work is continuing on the revisions.
"The technology part is something we are still working through," she said last night, "and [we are] keen to do that with the sector groups that are affected".
Half of schools would gain staffing entitlements.
Savings from class size changes are to be used to develop an appraisal system to help lift and reward teacher performance, with the aim of lifting student achievement.
Speaking for the alliance, NZ Educational Institute president Ian Leckie rejected the notion of the two-teacher cap over three years.
"People realise that after the three years, the rest of the teachers go ... the end result is quite clearly not a change in Government position. It is saying 'we will delay it a bit and then it will all happen'."
Mr Leckie said the alliance would reconvene if Ms Parata refused to meet them.
"A unified sector is a strong sector," he said. "There is strong community will behind us as well, and strong community outrage."
He said it was premature to discuss industrial action at this point and that was not possible unless a collective agreement had expired. Primary teachers and principals' agreements expire at the end of June and the secondary collective expires in October.
At her own press conference Ms Parata said some people had been irresponsible in "scaring parents" with suggestions that class sizes could double.
"I think that there's been quite a lot of heat and not so much light."