Speaking about the dangers of primary school-aged children using adult seat belts or badly-fitted booster seats, Dr Simpson said more awareness about keeping young passengers safe was needed.
It would make sense to have a law which ensured that children over the age of 5 used a restraint which was appropriate for their size and stage of development.
Progress had been made in getting children into appropriate restraints under the age of 5, with the back-up of the law, but there was a need to "maintain and improve on what we do, rather than say it doesn't look as much of a problem now".
Research showed that, in a crash, children too small for adult seat belts could sustain head, spinal and abdominal injuries which in some cases could be severe enough to kill them.
Often a small child would place the sash part of the adult belt under the arm or behind their back because they were uncomfortable. This meant that in a crash there was more upper body movement and the risk of the head hitting something inside the car.
Dr Simpson, who is deputy director of the University of Otago's injury prevention research unit, said with a properly-fitting harness there was less likelihood of the head being flung around.
A common reason parents gave for not putting children in suitable restraints was the difficulty of making the child comply.
When parents were in a hurry to get to work and a child was being difficult they might not be prepared to insist that there would be no trip until the child co-operated.
However, the bottom line was that they were responsible for that child's safety.
Dr Simpson said while designs of child restraints had improved it was not always easy to to get a really tight fit in cars.
The Plunket Society is able to advise people about child restraints through its car seat rental scheme.
A spokeswoman said it was important children were kept in a booster seat until they were tall enough for the vehicle lap sash seat belt to fit them correctly.
It was also important to ensure the booster seat fitted the vehicle and was properly installed and Plunket's child restraint technicians could help with that too.
The safest place for all car seats was the back seat, so they were away from airbags which released from car dashboards in a crash.
• The Paediatric Society of New Zealand has been agitating for a law change to better cover over 5-year-olds since 2008, saying then that New Zealand lagged behind the rest of the world. It described existing law which does not insist on booster seats for small children older than 5, as inadequate and out of date.
