The Children's Commissioner Andrew Becroft says banning smoking with children in the car will see 100,000 kids no longer dealing with second-hand smoke.
The Government has announced it will pass legislation by the end of the year, banning smoking in vehicles when children under 18 are present.
A select committee gave the estimation while unanimously recommending the law in 2015.
Children's Commissioner Andrew Becroft told Kate Hawkesby exposure to second-hand smoke has known negative impacts on health - including respiratory and bronchial problems, and cancer.
"[It] increases the risk of sudden infant death syndrome, it increases the risk of respiratory (bronchial) problems, the risk of cancer [and] meningococcal infection."
"If they weren't exposed to second-hand smoke, those issues would all drop, there would be huge health benefits to the system as a whole. It would be a really positive step forward."
He said a recent survey of year nine and 10 students showed around 20 to 30 per cent of them were exposed to second-hand smoke within the last week.
"If everybody stopped, that many children would benefit," he said.
In terms of policing the ban, Becroft said, "no one is saying it's an easy law to enforce".
"We don't speed because, at the end of the day, we know it's against the law."
"We don't use our cell phones, although we may be tempted to, because it's against the law."
"I hope a law like this will shift social norms [and] the police will be able to enforce it in the same way as when they stop cars for breath testing, seatbelt checks, license and registration checks, the can check for smoking as well, so I don't see it as an extra burden."
He said good education should be at the forefront of the law change.