Police are defending their four-month investigation into a principal who physically restrained a child in his class and stopped him running away from the school.
The principal was investigated between August and December last year but the police determined there was insufficient evidence for criminal prosecution.
RNZ has agreed not to identify the principal.
Tai Tokerau Principals' Association president Pat Newman, who spoke to RNZ on behalf of the principal, said the investigation was harrowing and took a severe toll on him.
In August last year, the principal removed the boy - believed to be 8-years-old - from a class where he was being disruptive, Mr Newman said.
"The mother, though, laid a charge of assault against the principal...assaulting her child when he restrained him and brought him over to the office."
The principal was then taken to the local police station and interviewed.
"He said he was grilled and grilled and grilled," Mr Newman said.
More than half of the interview was conducted in the absence of the man's lawyer, who had left the station, believing there was no basis for an assault charge.
However, the investigation went on for more than three months as police questioned the principal's friends and associates about his character.
In early December, the police determined there was insufficient evidence for criminal prosecution.
The investigation left the principal an "absolute mess", Mr Newman said.
"He started to question his own judgement about doing things, about never ever putting himself near such a thing again.
"He said to me, if the kids were going to be in danger then he wasn't going to stop it."
Child Protection Team Detective Senior Sergeant Kallum Croudis defended the investigation.
Complaints taken seriously and police carefully consider culpability, Sergeant Croudis said.
"Investigations such as this often take time and need to be thorough.
"When any force is used to restrain a child, a reasonableness test is applied taking into account the level of force and the potential harm that child could come to if allowed - as in this case - to leave the safety of the school grounds," Sergeant Croudis said in a statement.
New Ministry of Education guidelines for using physical restraint in schools were introduced in August last year.
Legislation allows a teacher, or authorised staff member, to use physical restraint if they believe there is a serious risk to the safety of the student or others.
Under the new guidelines, schools are now required to notify the ministry and the employer each time a staff member physically restrains a child.
The Ministry received 809 reports of physical restraint from schools in the first three and a half months of the new guidelines.
Mr Newman said the principal stuck to the guidelines.
"Everything was quite normal and this is how you would've normally have behaved and to be honest, it's what I would expect a principal to do, and that was within the guidelines," he said.
A Ministry of Education spokesperson said in a statement the new guidelines did not change a parent's right to make a complaint to the police.
"It is important to note the guidelines recognise teachers should use their professional judgement to decide how to manage challenging behaviour.
"The Education Act does not prohibit teachers from physically restraining children who are behaving dangerously in school.
"The guidelines were written to protect and support our children - children with challenging behaviour, as well as their classmates - and also to support and protect our teachers."