Police were not justified in using a police dog to bite a young offender following a pursuit, according to an Independent Police Conduct Authority (IPCA) report.
The 14-year-old was bitten as he ran away from the car he was driving in rural south of Hamilton two years ago.
The IPCA report noted the young person spent two days in hospital after being bitten on his leg by the dog.
It said the injury was "severe" and he may need skin grafts in future.
IPCA chair Judge Colin Doherty said the level of offending did not warrant a police dog being deployed to bite him.
"While there was no issue with police using the dog to track him, the offending that he had possibly been involved in was not at a level that warranted a police dog being deployed to bite him," he said.
He had taken his mother's car without her knowledge and driven off with five passengers.
When police signalled for him to stop the car, he fled and police pursued. The pursuit lasted for 18 minutes across 30km and at times reached 50km/h over the speed limit.
He eventually pulled into a dead-end driveway, narrowly missing a house, and ran away through farmland before being stopped by a police dog and bitten.
Waikato police district commander Superintendent Bruce Bird said they accept the IPCA findings that the use of a police dog was not appropriate in the circumstances.
"In this instance, staff were arresting an offender who had been involved in unlawfully taking a vehicle, fleeing from police and reckless driving," he said.
"These are fast-moving and dynamic situations that require an ongoing risk assessment by the officers involved."
The IPCA also found the initial decision to signal the car to stop was justified, as was the initiation of the pursuit and the young offender's arrest.
It found insufficient consideration was given to abandoning the pursuit due to the risks of the fleeing car's increased speed, the young person's reckless driving, and the number of occupants.