Smoking will be banned in cars when children under 18 are present, Associate Health Minister Jenny Salesa has announced.

Vaping will also be included in the prohibition and it will apply to all vehicles both parked and on the move.

"Public education and social marketing campaigns over many years have had some impact, but the rate of reduction in children exposed to smoking in vehicles is slowing. It is now time to do more by legislating," Salesa said.

She expected the Smoke-free Environments Act 1990 to be changed by the end of this year.

Once changed, police will be able to require people to stop smoking in their cars if children (under 18) are present.

They will also be able to use their discretion to give warnings, refer people to stop-smoking support services, or issue an infringement fee of $50.

According to research by ASH in 2014, 100,000 children a week are exposed to second-hand smoke.

The law change will come into effect by an amendment to the Smoke-free Environments Act 1990.

"First and foremost this change is about protecting children. However, it is also part of the Government's commitment to achieving Smokefree 2025," Salesa said.

"Too many New Zealand children, particularly Māori and Pacific children, are exposed to second-hand smoke in the vehicles they usually travel in," she said.

"Children are especially vulnerable to the harmful effects of second-hand smoke due to their smaller lungs, higher respiratory rate and immature immune systems.

"Second-hand smoke accumulates in vehicles, even with the windows open. It reaches much higher levels than in homes."

"The legislation will also be backed up with a new and innovative public education and social marketing effort.

"Ultimately, the focus of this change will be on education and changing social norms – not on issuing infringement notices.

There is likely to be strong support for the move with several surveys showing around 90 per cent support to ban smoking in cars with children present.

Australia, England, Scotland, Ireland, South Africa, parts of the United States, and most of Canada already have such bans.

Salesa made the announcement at the Weet-Bix Kids Tryathalon in Pt England, Auckland, alongside Children's Commissioner Andrew Becroft and the Ministry of Health's, director-general, Ashley Bloomfield.

Becroft welcomed the move citing wide health benefits.

"Many New Zealand children and young people are exposed to second-hand smoke in cars every day," Becroft said.

"Once this legislation is passed they will no longer be forced to inhale this chemical poison."

It would help reduce middle ear infections, cancer, sudden death in infancy, asthma and respiratory illnesses.

He also called on the Opposition to support the move.

"While some have questioned whether a ban can be enforced, there's really no issue.

"The police will be able to oversee it in the same way as they do the law on cell-phone use and seatbelts."