One in three low-decile primary school children are going years with undetected eyesight problems that could have "massive" consequences for their learning, new research says.
The charity, Essilor Vision Foundation, has found 30 percent of the 3000 low decile school students it screened had a vision issue they were not aware of by nine years of age.
The Massey University researchers who carried out the study said that raised questions about how children's eyes are being tested.
Last year, 180 year five and six pupils at decile one Rowandale School in Manurewa were among children who had free eye tests conducted by Essilor Vision.
Twenty pupils needed glasses and school principal Karl Vasau said parents were moved to tears as their children were presented with a pair at school assembly.
"It's a tool that these kids need to succeed," he said
"I can't image what it would be like for a child who can't see the board or read properly or can't catch a ball because they can't see it coming towards them... These 20 children have had that barrier or that thing taken away."
With poor sight going unchecked, problems could arise with performance, he said.
"If they can't engage in the learning ... some of them may disengage or may play up or sit back and wait and try and teach themselves later on.
"With a lot of these children, once they've fixed their eyes, away they go. It's amazing - it's like a whole new world for them."
With glasses costing more than $200, with additional costs such as the price of checks and getting to the optometrist, Mr Vasau said it was hard for many families to afford.
"For our families that amount of money for the little income that they do have takes a lot of planning and there will be a lot of sacrifices that will need to be made."
The Ministry of Health funds free eye checks for children before they start school and then again when they're in year seven, at age 11 or 12.
Massey University researcher Alison Kearney said a lot of children were going through the majority of their primary schooling without the glasses they needed and without being checked again.
"I think it raises questions about what's happening at the before-five health check and what sort of follow-up is there for those children who are identified at that check as requiring either further follow-up or corrective lenses."
If children had problems going unresolved it could have "massive" consequences, not just for school learning but social learning and behaviour, she said.
"The better their education, the better the health outcomes, the longer people live, the more income they earn over their lives. If we can address these issues very early on the advantages are enormous."
Dr Kearney said the before-five vision checks looked at issues like lazy eyes and squinting but did not screen for shortsightedness.
Most of the issues could be fixed by a pair of glasses, she said.
The Ministry of Health said its vision screening programme was in line with current evidence and best practice.
"There's little evidence to support frequent vision screening after school entry, or that undetected vision problems are a significant cause of academic under-performance," Child and Youth Health chief advisor Dr Pat Tuohy said.
Current testing focused on significant eye disorders like congenital cataracts and lazy eye, he said.
"Current evidence also suggests that contrary to the information provided, children with myopia typically do better at school than children with normal vision."
The ministry said if any parents or teachers had concerns about a child's vision or development they should get eyes checked.