Labour's education policy is a step backwards and would do little to help student achievement, the Government says.
The comment follows the announcement by Labour at its weekend congress that, if elected on 20 September, it would reduce student-teacher ratios in primary and secondary schools and train 2000 more teachers over the next three years.
The party said it would scrap national standards and the Government's plan to use good teachers and principals to mentor others.
Education Minister Hekia Parata told Radio New Zealand's Morning Report programme on Monday they were the same policies Labour tried when it was in government and student achievement only flat-lined, at best.
"That policy was Labour's policy the last time it was in Government and student achievement flat-lined at best. Investing in the quality of all teachers is more important than class sizes."
Ms Parata said the plan was flawed and Labour was seriously understating the costs of its policy.
Labour's proposed policy would reduce student-teacher ratios in years 4 to 8 in primary schools from 29 to 1 to 26 to 1 by 2018 and it has pledged to train 990 additional teachers.
The ratios in secondary schools vary from year to year, but Labour says it would train 910 more teachers and reduce the maximum average class size from 26 to 23.
Leader David Cunliffe admitted there was conflicting evidence about reducing class sizes, but nevertheless he felt reducing them was better for the education system.
Mr Cunliffe told Morning Report there was some literature, including an Australian study, which supported smaller class sizes.
"We believe this is a very substantial step forward and it is not done in isolation, it is done with raising the status of the teaching profession with creating better pathways for teachers."
The plan appears to have struck a chord with parents, according to informal questioning on Monday.
On Wellington's waterfront and civic square most parents told Radio New Zealand they want smaller classes for their children as that would allow teachers to give each child more attention.
However, some parents said they could see merit in National's plan to get the best teachers and principals helping others in order to raise the quality of teaching at all schools.
Disadvantaged have most to gain - academic
A Massey University education professor says children from disadvantaged backgrounds would benefit most from Labour's plans to reduce class sizes.
John O'Neill said most other children would be better off only if their teachers change the way they teach.
Professor O'Neill said research shows smaller classes have the biggest impact on very young children - where class sizes are already small.
Older children, he said, generally show little improvement in smaller classes, unless their teachers alter their teaching to suit a smaller group, and the research shows children from poor backgrounds do benefit.
Professor O'Neill said ideally, the next government will reduce class sizes in lower-decile schools and give teachers more time to improve the way they teach.