Police are investigating after a video emerged on social media showing a group of teens beating a girl at a West Auckland train station.

The cell phone video has gone viral.

The footage shows four to five teenagers beating up another girl waiting for the train at Ranui. It has wracked up more than 50,000 views and 400 shares on Facebook since it was posted on Tuesday night.

One girl punches the victim in the head, before the other girls join in, kicking and punching her.

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They continue to beat the girl after knocking her to the ground.

After about 30 second a male wearing a white t-shirt pulls the group off the girl and holds them back, shouting, "Enough, enough".

Laughing and yelling can be heard in the background.

The girl on the ground sits up and looks around after the male pulls the other girls off her.

AT spokesman Mark Hannan said the incident happened on Sunday afternoon and had been captured on CCTV.

A private security guard, who was on duty at the station at the time, called police.

AT was aware of the Facebook video and was holding the CCTV footage for police, he said.

A police spokeswoman said police were "very concerned" and were making enquiries into what happened.

Police asked anyone with information to contact Henderson Police Station or call Crimestoppers anonymously on 0800 555 111.