The incident happened on Friday at Edgeworth, in the NSW Hunter Valley, after a group of teenagers left a party about 10pm.

A copy of the video, that was filmed by a witness and initially posted on Snapchat, was sent to Newcastle Herald this week by outraged community members.

The video shows a young girl crying on the ground, trying to cover her face, as she is repeatedly punched and kicked in the head by another teenage girl.

At one stage the attacker stomps on the victim's head as another female, who is out of view, yells "bash her, bash her, bash her".

The victim, who is sitting on the ground and does not fight back, can be heard at the start of the video pleading for the return of her mobile phone just seconds before she is attacked.

"I will f--en beat you in the head and leave you right in this f--en grass. I don't give a f--, you are pissing me off," the attacker is heard telling the girl.

She then tells the girl she can't have her phone back as she plans to buy "cappies" with it.

The attacker then launches into repeatedly hitting the victim in the face before kicking and stomping on her head, as people are heard laughing.

A photograph of the victim seen by the Newcastle Herald after the attack shows her with a bloodied face, and swollen nose and mouth.

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The video attracted outrage on a host of social media platforms, with users urging witnesses to contact police.

Several teenagers contacted the Newcastle Herald about the assault after seeing the video on different social media platforms, describing it as "disgusting" and "terrifying".

A police spokesman said a 15-year-old girl was arrested on Wednesday and would be dealt with under the Young Offenders Act, which encourages police to divert young people away from court through warnings, cautions and Youth Justice Conferences.

He said investigations were ongoing.