Education Minister Hekia Parata says the Government made the decision to have bigger classes after it regained power at the last election.
The Government has been criticised for not signalling its intent to change class sizes before the election, with school sector groups now in uniting in opposition to the plans.
Sector groups are concerned the move will lead to a loss of teachers.
Ms Parata told Morning Report that the Government was working before the election on lifting student achievement by getting better data and assessment in Years One and Eight.
She said the decision to achieve that by increasing class sizes and investing in teacher quality was made after the election.
Ms Parata says there are currently 50,000 teachers in New Zealand and that number will remain static, as the Government works on improving teacher quality.
She also told Morning Report, she will meet different sector organisations separately this week, despite a request to meet them altogether.
The Labour Party says National would have lost support at the election if it had campaigned on its policy to raise class sizes in some schools.
National released its education policy in the last week of the election campaign saying it would make schools more accountable for their results and link funding to performance.
Labour leader David Shearer says he is certain National's polling would have suffered if the plans had been made public.