A grandmother has been hit with a hefty $2000 fine for not properly restraining children in her car.

She was among 40 drivers issued with infringement notices in a joint operation between Hamilton police and the Accident Compensation Corporation (ACC) today.

Sergeant Phil Ruddell, of the Hamilton strategic traffic unit, said the 38-year-old woman was stopped at a checkpoint in Fairfield.

"She was found to be driving with her 18-month-old grandchild in an untethered car seat in the front of the car, while her three- and four-year-old children were sitting in the back with just seat belts on and her six-year-old child was not restrained at all.

"To make matters worse, the woman was breaching the conditions of her learner licence by driving with passengers and she was behind the wheel of an unwarranted, unregistered car."

Police were not happy with the number of non-compliant drivers, although the majority have the chance to have their fines cleared.

"These checkpoints are about the safety of children under five being transported in cars.

"In most cases, drivers are given a 14-day compliance notice and if they can show within that time they've purchased or hired a car seat, or remedied a fault, then their fine is waived.

"The priority is about protecting the child, not revenue gathering and we hope getting such a notice will serve as a sufficient enough shock to get drivers to comply."

The objective of the joint checkpoints is to raise compliance levels of car restraint use for children under five from the current 65 per cent to at least 75 per cent within Hamilton city.

Waikato police were also worried about the behaviour of parents after a second underage driver was found behind the wheel of a car - this time taking small children to school.

Acting Waikato road policing manager Senior Sergeant Tim Anderson said the latest case involved a 14-year-old girl who drove up to a joint police-ACC child restraint checkpoint.

"It appears she had just dropped her younger siblings off (at school) for her mother," he said.

The child has been referred to Youth Aid while the mother has been charged with aiding and abetting an unlicensed driver to drive.

The penalty for such an offence is a $400 fine.

"This latest incident comes just 20 days after a 13-year-old boy was caught driving at 140km/h while his father slept in the front passenger seat," Mr Anderson said.

Police were "disturbed" by the prospect of how many other underage drivers were on the road.

"Children - and that's what these 13- and 14-year-olds are - don't have the cognitive skills to take in the big picture involved with driving and the consequences."