Plunket is warning ill-fitting or improperly installed car seats are putting children's' lives at risk.
The organisation was urging parents to check their child's car seat before heading off on holiday.
Plunket's national safety advisor Sue Campbell said while there was a high rate of car seat use in New Zealand, anecdotal evidence suggested many of them were unsafe.
She said spot checks often found the seat or booster was not fit for purpose, incorrectly installed, or not the right size for the child concerned.
All children under 7 years old must be restrained in an approved car seat or booster.