It is not clear how fast the car was moving but Ms Mills said it was slower than 20km/h.
She said the child suffered minor injuries in the incident but did not need medical treatment. She was pleased members of the public came forward with information for police.
"It's great that bystanders were so quick to tell us about the incident."
One person who was at the scene shortly after the incident told the Bay of Plenty Times he was shocked a child was able to get out of the backseat of the car.
He said the incident sounded "quite dramatic" and that the child was lucky he did not suffer serious injuries.
Tauranga Plunket child restraint technician Kerri Harnett said she was surprised to hear about yesterday's accident.
There was no excuse for a child to not be properly restrained in a car, she said.
"So many places out there are willing to help you so your children are properly restrained. You can rent or pick up car seats quite cheap.
"So there is no excuse for children to not be properly restrained."
She said there was still a high number of Tauranga children which were not being put in booster seats while in cars. "It is something we see every time we do a checkpoint," she said.
"I don't know if some people realise that it is illegal for a child to not be restrained while in a car.
"It is up to the adult, be that a parent or caregiver or relative, but it is up to them to do everything they can to look after that child and ensure it is safe," she said.
"When it comes to cars it is the responsibility of the driver to make sure that child is safe and is in a car seat."
Ms Harnett said there was information about the requirement surrounding car seats.
"Come to us at Plunket. We can actually show you what you need to do. People can also check out the NZTA website which tells you the requirements."
