Kiwi kids have slipped again in the latest international survey of maths and science.

New Zealand students' scores have dropped sharply in both subjects in Year 9, the first year of high school - with the lowest score for algebra.

A maths education expert, Dr Jodie Hunter, says part of the reason is that NZ schools stream students into ability groupings much more than most countries, excluding students in the lower groups from ever being exposed to harder problems.

"At some intermediate schools we work in, when we first go in the kids have never done fractions because we've had them in ability groups where they have just done addition and multiplication and never been given access to higher-level mathematics problems," she said.

Dr Jodie Hunter is co-director of Massey University's Centre for Research in Mathematics Education. Photo / Supplied

"For some children who have been put into low-ability groups, they just don't do algebra because the focus is on doing addition and subtraction, and that leads to the point of disengagement because they never see the challenge or the joy in mathematics."

The latest Trends in International Mathematics and Science Study (Timss) shows that NZ students have slipped slightly to 40th place out of 58 countries in Year 5 maths, down from a score of 491 in the last survey in 2015 to 487, based on a global average of 500 when the surveys started in 1995.

There was a similar slight drop in Year 5 science, from 506 to 503, but this was still above the global average.

But there have been sharp drops in Year 9 both in maths (down from 493 to 482), where we rank 23rd out of 39 countries, and in science (down from 518 to 499 - below the 500 mark for the first time).

Year 9 maths scores were weakest in algebra (464) and geometry (477), holding slightly higher for number (483) and data and probability (496).

In terms of "cognitive domains" for maths, NZ students scored lowest on "knowing" (468, down 20 points), and better on "applying" and "reasoning" (both down 7 points to 486).

Only 19 per cent of Kiwi Year 9 students could work out a simple algebraic problem where they were given the formula (2v + v squared) divided by 20, and were told that v=20. (The answer was 22).

Only 19 per cent of NZ Year 9 students got this problem correct. Photo / Supplied

Australian students did almost twice as well on the same problem - 37 per cent got it right. The global average was 35 per cent correct and Singapore topped the world with 73 per cent.

Kiwi Year 9 science scores were lowest in chemistry (482) and biology (498) compared with physics (502) and earth science (510).

As in maths, Kiwis scored lowest in science on "knowing" (480) compared with "applying" (503) and "reasoning" (510).

Ministry of Education Chief Science Adviser Professor Stuart McNaughton said the ministry was reviewing the curriculum and was considering specifying more content knowledge in areas where we scored badly, such as algebra and geometry, especially in the upper primary and intermediate years where NZ teachers are still generalists.

"We have got teachers saying they are finding teaching maths and science in the upper primary school years difficult," he said.

"And we have got a content knowledge problem - basic knowledge of maths and science. It's in the Timss data, and we also have it from the other assessments."

He said the answer might lie with using more maths and science specialists.

"We need to be better targeting what it is that we need - having more specialist maths and science teachers in upper primary," he said/.

"The number of kids who have access to specialist teaching in maths and science is way less than the OECD average. It's very small."