A police investigation is under way into the alleged assault of a 15-year-old boy in Hawke’s Bay by a police officer.

In the early hours of Sunday, March 16, a stolen vehicle was found crashed in Napier and four young people who were fleeing the scene were taken into custody.

The mother of one of them said on Facebook her son had owned up to “doing things he shouldn’t have been doing”, and was abused by police during the arrest and while under restraint.

She alleges his face was smashed into the ground, his jaw was broken in half, and he was punched in the head. She said it took a shift change at the police station for an officer to realise how injured her son was and take him to Hastings Hospital where he was sent to Wellington for surgery.

X-rays posted on social media by the teenager's mother. Photo / Supplied

Police told RNZ they were aware of the allegations and, while in custody, one of those involved complained of pain and was seen by a police doctor and then referred to hospital for further treatment.

“An investigation is now under way to establish the full circumstances surrounding the incident and how the young person sustained his injuries.

“Police are following up with the young person’s family and will be liaising with them directly,” police said.

The Independent Police Conduct Authority has also been notified.

The boy’s mother posted on social media about the incident.

“I’m mad, angry and keep crying that this has happened to my child from a member of the police force... assaulted and refused medical assistance for three hours,” she said.