The Government's decision to back down on plans to increase class sizes was driven by Education Minister Hekia Parata, Prime Minister John Key says.

"It was her decision but I was one of a small group of ministers that got on a conference call and went through our potential options," Mr Key told TVNZ's Breakfast programme today.

Mrs Parata initiated the conference call to address the backlash over the education policy announced as part of the Budget.

Mr Key was in Europe when Mrs Parata announced last week the Government would not go ahead with the policy.

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He admitted today that communicating information about the policy to change student-teacher ratios and how the Government would mitigate the impact on the worst-affected schools had not been handled well.

He said if the Government had gone ahead with the policy it would have intensified public opposition and the decision was driven by the debate with parents being lost.

"What it risked doing was causing months of industrial action, huge amounts of anxiety from parents and children and you've got to ask yourself is if that's really worth it and I think the conclusion we drew was no."

Mr Key defended the advice the Government had been given over class sizes.

He said the Government was advised that money would be better spent on improving the quality of teaching and not raising the number of teaching beyond the 52,000 teachers and principals currently employed.

"In the end the advice we had was having built up the stock considerably, it's better to spend it on the teaching quality, and that's a fair enough argument."

Mr Key said the Government was still committed to improving teacher quality but would find a new way.

Labour education spokeswoman Nanaia Mahuta said Mrs Parata should spell out her plan for improving teacher quality and student achievement.

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"This shouldn't be difficult. The minister has trumpeted this as her primary goal," she said.