A young woman is shown approaching a teenage girl who appears to be in school uniform, as another person follows along with a camera phone to catch the fight.

Police have arrested a 15-year-girl over a brutal beating of a schoolgirl that was captured on video.

Kaikohe police said on Friday that they had arrested the teen following an assault that happened at the bus depot in the town on Tuesday.

The prolonged assault was witnessed by a large crowd of bystanders, before a member of the public intervened.

Would you step in to help? Share your stories, photos and videos. Contribute

The alleged offender has been charged with assault and was due to appear in the Kaikohe Youth Court on Friday.

Police said they were also investigating the roles of people who encouraged and aided the alleged offender with a view to laying further charges.

In the video, which was posted on Facebook, onlookers stand around - many laughing and cheering - for about a minute before a pair of adults cautiously intervene.

A young woman is shown approaching a teenage girl who appears to be in school uniform, as another person follows along with a camera phone to catch the fight.

Without warning, the aggressor launches into a flurry of punches, knocking the schoolgirl to the ground, before kneeing and kicking her in the head.

The person filming the video makes no apparent attempt to stop the assault, nor do several witnesses.

Adult bystanders are eventually seen arriving on the scene and separating the girls. The aggressor in the fight then lunges for an adult male who tried to intervene, before landing a final savage kick to the back of the other girl's head.

Kaikohe Senior Sergeant Brian Swann said the victim of the attack had suffered injuries to her face and head.

"The victim and her family are understandably very upset and shaken by this vicious, unprovoked assault."

Swann said it was of great concern to police that the video of the assault had been posted online and widely circulated. "The sharing of this video indicates an acceptance of this type of thuggish behaviour and a glorification of the offender while causing further harm to the victim.

"Police would like to ask everyone who has shared this video to remove the post and stand up to oppose violence in our community.

"Kaikohe police would like to thank the members of the public who came to the aid of the victim of the assault, however are concerned that so many stood by and watched the attack take place. This type of attack has no place in civil society."