The New Zealand Initiative says some teachers are not necessarily qualified in the subjects they teach.
The business think-tank has this week released the first of three reports on education.
It has analysed past research and conducted interviews for the first report, which looks at why New Zealand needs to strengthen its teaching profession.
It says about 86% of the country's teachers have at least a bachelor's degree.
But it says teachers are not necessarily qualified in the subjects they teach.
About one-third of year-nine secondary students' maths teachers don't have a mathematics qualification.
Of all teacher vacancies, 19% are in mathematics.
The group will publish two more reports in the coming months.
"We talk about in the report how there's been a series of reforms over the last 50 years that have meant to be the silver bullet for improving education," research fellow Rose Patterson says. "Much of it has actually been very ineffective because not a lot of it's actually focused on improving the teaching profession."
