Fitness New Zealand part-time worker Danny de Hek said the youths, who are known to other staff members and Papanui Rd businesses, often try to damage shop signs.
De Hek said he went outside about 10am to tell the teens to stop vandalising their property - and one of them pushed him.
The three youths left the area but returned later that day. They came into the shop and De Hek started filming them so he could trespass them.
One of the boys tried to hit De Hek's phone out of his hand and broke his glasses.
"They just punched me with no warning," De Hek said.
"And I just thought 'wow'. And I thought he's just a kid and you kind of give them the benefit of the doubt, but this sort of got serious."
"And then his mate came in ... he was dressed in black with a balaclava and he had his hands down low.
"I didn't want to lose eye contact with him because I didn't know if he had a knife or anything.
"And then he told me not to pick on his friend who just punched me - and then he punched me as well."
His attacker later contacted him and asked him to take the video off social media, but he declined.
"When I was talking to him, I asked him 'can I speak to his mother?'
"And unfortunately, she said that she's got zero control over her 14-year-old son.
"They broke my glasses, and they haven't offered to pay for the glasses, nor have they even offered to say sorry for what has happened.
"He was more concerned about getting the video off to save his bacon."
A police spokesperson said they are investigating the incident.
“Since the assault, police have identified the young people involved and inquiries are ongoing,” the spokesperson said.
- Watch De Hek's video at www.youtube.com/watch?v=sdPxRZWtYiI
- By Geoff Sloan, made with the support of NZ On Air
