Health researchers want the government to think about making car seats compulsory for children over the age of seven.
A law change made in 2013 means children need to be in an appropriate child restraint up until their seventh birthday, or up until their eighth birthday if a restraint is available in the car.
Auckland-based researchers surveyed parents whose children were in hospital, and their findings, published in the Medical Journal, found that over the age of six the use of car seats declined.
"The 2013 amendment to child restraint legislation increased the use of child restraints in the age range stipulated by the law, but child restraint use in older children remains suboptimal," they said.
International studies have shown booster seats reduce the risk of injury in older children.
The researchers said more work could be done to look at the barriers parents face using child restraints, and the impact of extending the car seat rules to older children.