A Kiwi teenager was stomped and kicked by Tongan police wearing steel-capped boots, after a victory joyride in the Pacific island nation turned brutal, his family alleges.

A top-level investigation is underway after schoolboy Francis Norris was in a group of 11 - including a brother of former Warrior and Tonga wing Manu Vatuvei - allegedly assaulted by up to five policemen during exuberant rugby league post-match celebrations last month.

The 16-year-old league fan was left bruised and lame after a metal torch was thrown at his legs, then punched and kicked as he was placed under arrest.

Norris's aunt Saffron Norris is livid and says the shocking incident has not only been traumatising but left him nursing wounds so bad he has been off school for the past fortnight.

The family has lodged a complaint with the Tongan Police Standard Unit over the brutal treatment.

A spokeswoman for Tonga Police confirmed the matter was now under investigation.

New Zealand Embassy staff in the Pacific kingdom are also working with the family.

Norris, an Auckland website designer, has extended her holiday in Tonga to care for her nephew.

She said the group of sports fans were caught up in the revelry after Tonga's victory over Scotland on October 29.

They crammed in and on a vehicle and travelled into the centre of Nuku'alofa to join scores of other jubilant fans blaring horns and shouting "Mate Ma'a Tonga".

As they returned home to the village of Fangaloto they were stopped by a police officer and told to sit inside the vehicle, she said. They were stopped a short time later by police for a second time, who, after approaching the packed car, started pelting the stunned fans with "blind shots".

The 16-year-old told community blog site Tonga Hub he was frightened and ran. But his escape bid was short-lived when a police officer threw a metal torch so hard when it struck his leg he fell to the ground.

He also claims he was beaten about his body and head by up to five police officers in a "continual brutal assault".

"They stomped on my head repeatedly and kept punching my head and body", Norris told Tonga Hub.

"I kept saying 'toko fakamolemole, toko fakamolemole' [I'm sorry brother, I'm sorry brother] but they wouldn't stop. I was hurting everywhere. I was scared for my life and I thought I was going to die."

He claimed he was dragged to the police vehicle and put in the back seat where the assault continued.

At one point he claimed he was dragged out of the vehicle, had his head rammed onto the side of the vehicle, then thrown on the back of a police ute where he was put in handcuffs.

"I ran for my life inside, heading towards the cells to join the other boys. I was so weak and badly hurt I did not know the rest of what happened and who was around me. I knew I had to be close to my friends who were already inside the cell. I just wanted to survive."

The Toloa College pupil was now recuperating at home as family, friends and the wider community rallied around him.

His family say they are "utterly shocked and disgusted" with how the police handled the group.

"It was entirely uncalled for. It wasn't necessary for them to use that level of force and we don't want this to happen again," said the injured boy's aunt.

The officers were taking the law into their own hands, she said, and she wanted to make the public aware that this disturbing level of unprovoked violence was still being meted out by police - five years after a Kiwi was killed in a Nuku'aofa police cell.

Two officers were convicted of manslaughter and another of assault over the fatal beating of South Auckland policeman Kali Fungavaka, in Tonga for his father's funeral.

Norris' aunt said the group of league fans was charged with making a disturbance and breaking the Sabbath law.

In a court appearance last week, the younger members of the group - who ranged in age from 16 to 47 years - were let off with a warning. Three were fined.

Norris and another teenager lodged formal complaints with the police.

Tonga Police media officer Sia Evans told the Herald an investigation was underway into the allegations of police assault after the family of the Kiwi teenager lodged an official complaint.

A spokeswoman for the Ministry of Foreign Affairs and Trade confirmed staff were helping an injured Kiwi in Tonga.

"The New Zealand High Commision in Tonga is providing consular assistance to the family of an injured New Zealander," she said in a statement.

"Due to privacy considerations, the Ministry of Foreign Affairs and Trade will not provide further information."