Health organisations and public health experts say there is no logic to the government's refusal to ban smoking in cars around children.
In response to a petition, the Health Select Committee late last year recommended smoking in cars be banned if anyone in the vehicle was under 18 years old.
The government has rejected that, saying there was "no point putting a law in place that's likely to be flouted" and "present initiatives are sufficient".
Smoking in cars when children are present is already banned in the UK, South Africa, Australia, most of Canada, and parts of the US.
"It's a total mystery to me why they'd reject this evidence-based effective policy, which has got plenty of international precedence," said ASH spokesperson Robert Beaglehole.
Health Select Committee chair and National MP Simon O'Connor said he believed a ban would send a message to the public that smoking around children was unacceptable.
"The committee wanted to make a statement, if you will, that yep it could be difficult to enforce, maybe it won't change huge numbers of people's behaviour, but if we're making the statement that it's not acceptable, maybe that will begin to help and add to what's already being done," Mr O'Connor said.
A 2012 survey of more than 28,000 New Zealand Year 10 school students found 23 percent had travelled in a car in the past week with a smoker.
For Māori, that figure was 40 percent.
A 2014 Ministry of Health funded survey found 97 percent of New Zealanders supported banning smoking in cars when children were present.
Three other surveys all returned with support above 90 percent.
Cancer Society chief executive John Loof said he was really curious to know the logic behind the government's decision, because it did not stack up.
Mr Loof said there were no current initiatives focusing on smoking in cars.
"These are simple measures to protect the health of our kids," he said.
The Stroke Foundation also questioned the decision, saying it did not make sense.
"They've just set up the Ministry for Vulnerable Children but this was a huge opportunity to protect children that are vulnerable to second-hand smoke and they've done nothing," Stroke Foundation spokesperson Julia Rout said.
But Associate Health Minister Nicky Wagner said any measures the government took to reduce smoking in cars needed to be effective.
"There is no point putting a law in place that's likely to be flouted. Unless there's clear evidence of smoking while a child is in a car, it's difficult to enforce. Education and media campaigns may be more effective."
Ms Wagner said the government already had programmes in place to encourage New Zealanders to stop smoking and those were getting good results.
"This is where we need to continue to focus our efforts in order to achieve our goal of a smokefree New Zealand by 2025."