The Education Review Office has called for urgent action to improve maths teaching in primary and intermediate schools.
A report published on Thursday shows that half of New Zealand's primary schools could make a big difference to their students if they changed their curriculum.
The report says schools need to change what they teach so they better meet the needs of all their learners, especially those who are struggling.
Only 11% of the 240 schools the review office visited last year were good at reviewing and designing their maths curriculum.
The report says only a quarter were good at using students' results to inform teaching and spending decisions.
And it says most schools were using long-standing approaches to helping struggling students improve, but with little evidence that those approaches were effective.
It also criticises the widespread use of teacher aides to help low achievers, saying schools are using their least qualified staff to work with the learners who need the most expert teaching.
The report recommends schools evaluate and improve their maths programmes and that the Ministry of Education do more to help them with that work.
The report comes two months after a study involving 60 countries revealed New Zealand 10-year-olds have got worse at maths in the past five years and are performing below the international average.