A police spokesman said staff were investigating the fight. "A complaint was made to us. Three youths are helping us with our inquiries," he said.
Police were also trying to have the video removed from social media.
The Herald has chosen not to name the victim, or publish the video because of its graphic nature.
It starts with one boy holding the 18-year-old while another punches him, knocking him to the ground. A third boy then kicks him in the head.
The victim is then picked up by his clothes and dragged a short distance before being thrown onto the ground.
He is seen cowering as the other teens approach him and threaten to hit him. They then turn and walk off.
The video was filmed by a friend of one of the attackers and has had more than 260,000 views on Facebook and 1000 views on YouTube since it was posted.
The family spokesman said the attack happened after the 18-year-old accidentally injured a female friend while they were playfighting on the beach.
"He accidentally knocked her with his elbow and bruised her eye. He was mortified, and took her flowers and everything to apologise.
"One of the guys filmed it [the violent attack] on his phone and sent it to the girl and said, 'We've fixed it for you.' She is absolutely horrified. Two wrongs don't make a right."
It is understood some of the attackers were from a youth gang in Waihi, and one knew the 18-year-old.
The spokesman said the family and the victim were extremely upset that the video clip had gone viral.
"It's not good. We just want to get the video removed and get on with it, and let the police do their thing.
"There has been a lot of support from the community and from friends and family up and down the country."
Viewers have expressed immense disgust at the violent attack, calling it "gutless" and "cowardly".
