People often had more than one vehicle and when seats were switched between cars regularly, sometimes they weren't set up right.
Horizons funds Plunket to carry out the checks.
Plunket recommends putting any child under 148cm in a car seat while the law requires anyone under 7 to be restrained in a car seat.
Mrs Leitao said people could always seek advice from Plunket.
Plunket education and advocacy co-ordinator Veronica Fieldsend said one difficulty was that New Zealand imported seats with standards from varying countries.
"It is confusing because there are so many different seats that are allowed in New Zealand. It's just a matter of education," she said.
"This is your child's safety."