An Auckland mum lashed out and assaulted a teenage girl who had been bullying her daughter - grabbing her by the hair and hitting her repeatedly in the face.

But Kingsland woman Nicola-Jane Jenks has avoided a conviction, with a District Court judge saying she had made a "bad decision" which should not impact on the rest of her life.

And Jenks says she regrets the incident but, it was the "consequence" of the ongoing bullying.

In May Jenks assaulted a 17-year-old near Mount Albert Grammar School after receiving a distressed phone call from her scared daughter.

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Her child had been bullied for some time and it had a huge effect on the teenager.

On the day of the assault Jenks effectively snapped and fought back in a bid to ease her child's suffering.

She was charged with common assault and at her first appearance in the Auckland District Court, pleaded guilty.

At sentencing on Thursday Judge Tony Fitzgerald said Jenks had made a bad decision under circumstances that would have tested any parent.

"You quite rightly feel ashamed for the way you acted," he said.

The court heard that two years of physical and verbal bullying had foreshadowed the way Jenks had acted on May 14.

Judge Fitzgerald said that In October 2016, the teen Jenks assaulted had thrown a bottle at her daughter, striking her in the back.

Photographs show a bruise on the collar bone "that suggests it was thrown with some force" - an issue Jenks took up with the school and police.

Over time the bullying took a toll on Jenks' daughter - her hair was thinning, she experienced panic attacks, and her attendance was down at school because of her fear of going, the court heard.

The assault on May 14 was the first time Jenks had met the 17-year-old involved in some of her daughter's ongoing bullying.

Words were exchanged and the victim told Jenks to "f*** off" and threatened to hit her, the court heard.

"What you then did was inappropriate," Judge Fitzgerald said.

Jenks dragged the 17-year-old by her ponytail and hit her several times with an open hand.

The teenager suffered bruises to the left cheek and scalp.

She also reported to police vomiting and spending hours in hospital at the time to ensure there were no internal injuries.

In her latest victim impact statement the teenager said she no longer liked to be anywhere alone and felt humiliated that the assault occurred in front of her peers.

She had sought counselling in the wake of the attack.

Judge Fitzgerald said an aggravating feature was that the assault happened in the vicinity of the school where both of the young women should have felt safe.

Both girls felt they no option but to leave the school after the assault, the court heard.

"You are 50 years old and you should have handled it a lot better than you did," said Judge Fitzgerald.

He explained there were many factors to be considered in Jenks' favour.

Jenks had no previous convictions, had entered a guilty plea, had engaged in both volunteer work and counselling, and had offered to partake in restorative justice.

"Your remorse is deep and genuine."

Judge Fitzgerald granted Jenks a discharge without conviction, saying a conviction would make it difficult for her to travel to other countries and would complicate similar applications - consequences out of proportion to the offending.

Jenks' lawyer said yesterday she did not want to comment on the case as she felt the matter had been dealt with by the court.

After her first court appearance, in a previously unpublished interview, Jenks said she regretted her actions - but said the problem of bullying in schools had to be addressed.

"Of course I do [regret the incident].

"But my daughter was bullied for two years and this is the result of it.

"It's about bullying in schools and it just has to stop. This is the consequence of it."

MAGS principal Patrick Drumm was aware of the incident but reiterated it did not happen on school grounds.

"We do not tolerate violence, harassment or bullying under any circumstances," he said.

"The school does not condone the assault by Nicola-Jane Jenks."

Drumm said MAGS considered the health and safety of its students and staff as its primary responsibility.

He was unaware of any bullying of Jenks' daughter during school time and understood that the conflict arose outside of school, between two families, during the weekends.

"Had any bullying been alleged at school, during the school day when we do have jurisdiction, the school's robust pastoral care and support systems would have been immediately put in place to provide a safe learning environment for the student concerned," he said.

"We would also have investigated the allegations and dealt with the issue comprehensively."

He said the victim and Jenks' daughter were former students.