"Staff will be using their discretion while the new law is introduced, with a strong focus on education.
"However, there will be no tolerance for people who put children's lives at risk by wilfully ignoring the law or failing to restrain a child where restraints are available."
New Zealand Transport Agency (NZTA) road safety director Ernst Zollner said child restraints used in New Zealand must display standards markings to show they are approved and safe to be used.
Baby on the Move Auckland franchise owner Kylie McCracken said they hadn't noticed a large increase in sales while parents prepared for the law change.
Associate Transport Minister Michael Woodhouse said the rule change was agreed by Cabinet in 2012 and signed in June this year.
West Auckland mother Kirsty Mew said she bought a capsule for her two children Hunter, 4, and Charlotte, 7, when they were babies.
They then transitioned to car seats with help from a Plunket initiative, before moving on to booster seats.
She said the law change was "great". "Kids, they're only little, I don't think you can put a price on their life. If it makes it safer, then why not?"
Ms Mew estimated she had spent around $330 on child seats over the past seven years.
Plunket national child safety advisor Sue Campbell said some parents were eligible for child seat grants through Work and Income and Plunket ran a cheaper child seat rental programme for those on low incomes in some areas.