There is little evidence that requiring schools to teach maths, reading and writing for an hour a day will improve children's achievement, a briefing paper shows.

In fact, the document says international tests indicate more teaching results in worse performance.

The policy was "likely to have a positive impact on student achievement" if combined with guidance on quality teaching methods, the Ministry of Education's advice to education minister Erica Stanford.

However, elsewhere it says the policy "may" have a positive effect and its impact on achievement across the entire curriculum, including other subject areas, could be both positive and negative.

The Cabinet paper containing the advice also says the policy would essentially cost nothing.

The paper says there were few studies or reports about the effects of mandated teaching hours on student achievement in language and maths.

It says a study in Italy found mandated teaching hours improved student achievement in maths, but not in language.

However, international tests of reading and maths showed student achievement declined very slightly as teaching time increased.

It says international studies showed New Zealand teachers of Year 5 pupils taught maths for an average of 51 minutes per day, one minute short of the international average, and reading for about five-and-a-half hours per week or about 23 per cent of teaching time which was much higher than the international average of 16 per cent.

The report says teachers spent a further three hours per week on other language-related teaching, equivalent to 10 per cent of teaching time, which was in line with the international average.

But the paper says some schools were not teaching the subjects for an hour a day and the policy could have a positive impact on student achievement.

"Many schools across New Zealand routinely schedule minimum class time each day for the basics.

"But this is not consistent - which means each student over time and all students across the system are not getting the same opportunities to learn the basics.

"There is an opportunity to ensure students get access to regular and quality teaching about key matters, such as reading, writing, and maths, as this could help lift student achievement."

The report says mandating an hour a day for each subject carried risks.

"Teachers may need to spend additional time on a subject just to meet the time requirements, even if this does not fit with the specific needs of their students.

"The easier ways to meet the time requirement may not necessarily be the most effective teaching for these subjects or for all students," it says.

"Progress in other learning areas could be negatively impacted, if there is only an hour or two for other learning areas (e.g., technology, science)."

Erica has previously told RNZ the policy was designed to ensure consistency across schools.

She says most were already teaching the subjects for an hour a day and the policy would act as a reminder that schools needed a "laser focus" on reading, writing and maths.

The Cabinet paper including the ministry advice says the rule would not apply to specialist schools, which work with children with disabilities, until 2025.

It says consultation with kura kaupapa Māori and kura-a-iwi was required before the policy could go ahead for them.

"Kaupapa Māori education providers may want a different approach, and it is their expectation that given the Treaty principle of partnership, we should engage with them in advance of any decisions affecting their kura.

"In initial conversations, Te Rūnanga Nui o Ngā Kura Kaupapa Māori o Aotearoa and Ngā Kura ā Iwi o Aotearoa (the two main representative bodies of kaupapa Māori education) have raised the need for more time to work with the government to design a mutually agreed accountability framework," the Cabinet paper says.

"Given this and the potential risk of legal challenge on Te Tiriti grounds, I recommend that the teaching time requirements come into force in term one 2025 for specified kura.

"This will allow time for officials to engage with kaupapa Māori education providers in advance of these decisions affecting their kura."

The paper says the policy had no direct financial implications and the Ministry of Education's work on it would be covered by its regular funding.

The paper says testing indicated only 35 per cent of Year 8 students were writing at their expected curriculum level, 56 per cent were reading at the expected level, and 42 per cent were reaching the expected level for maths.

Results from the OECD's most recent Pisa study showed 21 per cent of New Zealand 15-year-olds were poor readers and 29 per cent were poor at maths.

Both figures were better than OECD averages but worse than New Zealand's previous performance.

- RNZ/ John Gerritsen