A police officer was almost in tears after stopping a four-wheel drive crammed with 13 people on Auckland's Northern Motorway - with a baby among eight unrestrained children travelling inside.

Constable Sheryll Pearce was astonished to find 13 members of an extended family packed into a seven-seat Nissan last month.

Describing the situation as "unthinkable" Pearce's response to a phone call from concerned members of the public could have averted a huge tragedy.

With two adults in the front seats the only ones wearing seatbelts, behind them were three women and eight children - one of which was a baby - squeezed across five seats and between rows.

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"When I opened the door, there was just this seething mass of unrestrained humanity. I just could not believe how many bodies were inside," Pearce told the Herald on Sunday.

"No one, except for the two people in the front, were wearing seatbelts. I was just shocked.

"There were eight children in the vehicle, six of them should have been in booster seats and two should have been in baby capsules.

"It's unthinkable that adults who have children they love could not care enough about making sure their children were secure. I was appalled."

Pearce had the family arrange for more vehicles to meet them in order to carry all the passengers, but not before one of the group questioned why she was so concerned.

"When these extra seats turned up, the adults didn't have a clue really how to secure the restraints properly and didn't know how to make sure the children were safely secured in booster seats," Pearce said.

"One person in the car, a mother, thought breast-feeding her baby was OK and got angry at me for being upset with them.

"I was nearly brought to tears by it, I was so angry and frustrated."

Out of a potential 11 tickets the group could have been slapped with - including unpaid road user charges and totalling $1650 - Pearce issued two - sending the group across the road to a car shop instead to buy appropriate child seating.

"I felt their money was better spent getting child restraints than fining them into the ground," she said. "I really hope what happened has made a difference to them in the long-term.

"If their vehicle had been in a crash, there could have been a lot of deaths, or very serious injuries. You;re talking about cars travelling about 100kp/h."

As Pearce's safety message hit home, she said a number of apologies were made to her by the family.

"The adults certainly got the message that I was pretty upset and cared about what was going on."

Pearce's handling of the situation has been lauded by national road policing manager Superintendent Steve Greally.

"She nailed the message home to them about their kids' safety," Greally said. "Too many infringements could potentially result in non-payment and no safety benefits. Her approach will ensure those kids are safer."

Waitemata district road team coordinator, Todd Moore-Carter, said road users must get the message about seatbelts.

"There were 328 road deaths in 2016 and 100 of those were not wearing restraints," Moore-Carter said.

"Please, put your seatbelt on. You do not want to risk a life of misery."