A primary school principal has labelled the government's class size policy a "slap in the face", saying the education minister must be "crazy" if she thinks it will help existing teachers or attract new ones.

Jan Tinetti earlier on Monday announced $106 million to put more 320 teachers into classes teaching years four to eight, dropping the average class size for those years from 29 pupils per teacher, to 28.

The goal was not expected to be met until 2025, Tinetti saying she's "not happy with the downward trends we are seeing in maths, reading and writing".

The election-year promise was met with incredulousness by the principal of one lower-decile school, who says it should have been a reduction of five - not one.

"I just think it's ludicrous," Finlayson Park School principal Shirley Maihi told Checkpoint.

"I think it's a slap in the face. And it certainly won't entice people to come into the teaching profession."

Tinetti says potential reductions in class sizes "over the long-term" would be looked at by a ministerial advisory group.

"For the minister Tinetti to think she needs more time… I mean, she comes from a principal background," Maihi says.

"She should know what the issues are… Surely we don't need another four or five years of people sitting around a table and talking about what the needs are. It's got to happen now."

NZEI Te Riu Roa, which represents more than 46,000 principals, teachers and support staff say the 1:29 ratio had been in place since 1996, and a review from 2000 recommended making it 1:25.

"Our members have been campaigning for a reduction in class sizes and improved staffing for many years. It was a key reason for our most recent strike on March 16," president Mark Potter says in a statement welcoming the new funding.

The National Party leader says he's sceptical about the government's ability to actually reduce class sizes and increase the number of teachers.

Christopher Luxon calls the proposal "a bit hard to stomach" on the back of Prime Minister Chris Hipkins being education minister for five-and-a-half years before Tinetti.

"If it was so important, why didn't he do it five-and-a-half years ago? It's something that we support obviously, but I'm very sceptical about this government's ability to get it done. The real challenges we have is that half our kids arrive at high school not where they need to be in terms of expectations - that's because we're not teaching the basics well, of maths, reading and writing, and setting them up for success."

National in 2012 proposed increasing the average class size to cut costs, but ultimately backed down. Labour went into the 2014 election promising to slash the ratio down to 1:26 for primary school classes, but was not elected.

National's flagship education policy this election is to mandate an hour of writing, reading maths every single day.

Luxon says this would address serious issues with literacy and numeracy in primary and intermediate schools.

Earlier on Monday, Tinetti noted the 2019 National Monitoring Study of Student Achievement (NMSSA) found in writing, at year 4 nearly two-thirds of students were achieving at the expected level, dropping to just 35 percent by year 8; and in reading, the drop was from 63 to 56 percent.

The most recent maths figures were for 2018, which showed 81 percent achieving at year 4, but just 45 per cent by year 8.

"We used to do it really well in this country, you know?" says Luxon.

"Fifty per cent of our kids showing up at high school not ready to go… two-thirds of our 15-year-olds failed the most basic maths, reading and writing test."

In 2018, Labour had only just come to power the year before. According to that year's NMSSA, year 8 students' results were higher than in 2013, and the difference for year 4s was not statistically significant.

For writing, the 2019 NMSSA found "no significant change in the proportion of students achieving at the expected curriculum level" between tests done between 2012 and 2015, and 2019, but "proportion of students achieving at the expected level in year 8 remains low".

Speaking to Checkpoint ahead of Maihi, Tinetti denied there was a teacher shortage - at least in primary schools.

"We've had teachers that are coming in from overseas and we've brought in 1000 of them so far over the last couple of years. Since 2017, we've increased by 3000 the number of teachers working within the system already.

"But we also know that we've got to train more teachers, and that's why we're looking at different ways of piloting how we train our teachers. And we've got those pilots up and running in Auckland."

She says the new policy "absolutely can" get the ratio in primary schools down to 1:28, but would not comment on existing negotiations with teachers on pay and conditions.

Maihi says it was "absolutely not true" there was no shortage of teachers in the primary system - saying she had three vacancies at present, but would not settle for just anyone.

"We need New Zealand-trained, good quality teachers. I'm sorry, there's very few out there, very few, and they don't seem to come to schools in lower-decile areas because of the perception… quality teachers are not available to many of our schools."

- RNZ.