Random roadside checks by Plunket and police have found up to 80 per cent of child restraints are installed incorrectly or not used at all.
"Some of the mistakes are minor and the child would be okay but some are significant and can endanger the lives of the children," says Plunket's national child safety manager Sue Campbell.
"Many parents are unaware and would be horrified to know they had the seat incorrectly installed."
Campbell says next month's new law requiring children to be in a car seat until their seventh birthday is a perfect opportunity for parents to make sure their carseat or booster is installed correctly and suitable for their child. Advocates argue some children should stay in carseats longer, until they are 148cm tall.
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Last week, the Herald on Sunday set up its own checkpoint outside Auckland's Pt Chevalier primary school and found some children incorrectly restrained - even under the current, less strict, law.
This comes as no surprise to carseat installer Trish Thorburn from NZ Child Restraints.
"The most common mistake is moving a child into a booster before they are ready," Thorburn says. "Some children reach the weight limit for a harness seat and, instead of buying one with a higher weight limit, they go to a booster with just an adult seatbelt."
Other common mistakes include using the wrong carseat for the height of the child, not using tether straps and not securing the seat properly to the car.
Dr Liz Segedin from Starship Children's Hospital says a child is admitted to intensive care every month with injuries caused in a car accident where they were not properly restrained. At least one a week is seen on the general ward.
"The injuries of concern are serious head injuries which are the risk with any form of improperly used car restraint," she says.