Children in their first two years of primary school will be legally required to be restrained in car seats, but experts say it's height that matters.
If children were restrained until they were 148cm tall, it is estimated five lives a year could be saved and 304 injuries prevented.
The law change, due next year, would make it mandatory for child car restraints to be used until a child is 7-years-old - adding two years to existing child restraint rules and bringing New Zealand into line with other countries.
Dr Michael Shepherd, clinical director of Starship's emergency department, said raising the mandatory age for being properly restrained from five to seven would reduce the number of injuries sustained as a result of children using adult seatbelts.
Dr Shepherd welcomed the law change, saying it was a major step forward, but said New Zealand should follow the lead of other OECD countries to extend the compulsory use of child restraints in vehicles until a child is 148cm tall.
"The evidence is that you should be 148cm to fit an adult seatbelt. In my mind that seems like the right thing to tell people to do, but whether you do that by legislation or some other way is part of the debate.
"We should be very clear to people that unless your child is 148cm tall that you're not ideally restrained while wearing an adult seatbelt.''
Dr Shepherd said children who were not properly restrained in cars and were in car accidents would arrive at Starship's emergency department with head injuries, serious abdominal injuries and spinal injuries which often result in paraplegia.
"The advantage of this legislation is people will use booster seats a bit longer for their children.
"It will decrease some of the serious and permanent injuries and disabilities that we see as a result of seatbelts.''
Plunket, Safekids and the Motor Trade Association agree that children aged over seven are not tall enough to be safely restrained by an adult seatbelt, and say the law should go further.
Plunket's National Child Safety Advisor Sue Campbell said the changes were a step in the right direction but said a height rule should be in place.
"Research shows that a child should remain in a booster seat until around 148cm tall.
"For the majority of children this is somewhere between 9 and 12 years of age. Adult seatbelts are designed for adults. They don't fit children properly until they're at least 148cm tall,'' said Ms Campbell.
In Switzerland children are restrained until they are 12, or 150cm tall, and the United Kingdom has a 135cm height policy.
Safekids' Director Ann Weaver said the height restriction was important because children do not fit adult-sized seatbelts.
She said seatbelts used without booster seats rest on the child's neck and high up their stomach. If there is a collision, the children will sustain serious injury or death.
Associate Transport Minister Simon Bridges said the age will not be raised at this stage but a consultation process over the next year will look at issues to do with taxis using restraints, and big families in small cars.
"I think it's likely that parents will continue to use the booster seats for them as they get older,'' said Mr Bridges.
He said New Zealand's road toll had reached an all-time low last year with 284 deaths, down 25 per cent on 2010 - the lowest in the 50 years since records began.