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.
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.