A policeman sitting in his car with his family warned a gang member not to do anything stupid moments before being punched in the head.
The officer's wife and daughter shouted in panic as former Satan's Slaves gang member Leslie Frederick Tofts, 39, laid into him through the driver's window.
The attack happened this month in Upper Hutt.
It was recorded from the back seat of the car by the senior constable's 15-year-old daughter. The video footage shows Tofts unloading punches on the policeman, while the daughter yells at her dad to drive away and his wife shouts "ring the police".
Tofts pleaded guilty in the Hutt Valley District Court last week to assaulting the officer and was sentenced to 110 hours' community service, a sentence described by the Police Association as an insult.
The assault followed a sustained period of aggression and intimidation towards the officer by Head Hunters motorcycle gang members, according to the case summary of facts.
The officer, who did not want to be named, had "been having ongoing issues with Head Hunters gang members intimidating and harassing him at his home address".
A Head Hunters member had that day already tried to intimidate him by slowing down on his Harley-Davidson motorcycle and eyeballing him as he washed his car in his home driveway. Later, the same member stopped his bike in the middle of the road in front of the officer's car and yelled at him.
Eventually, confronted by a bunch of Head Hunters and their associates, the officer pulled his car over. Then Tofts assaulted him.
"(Tofts) was yelling at the victim," the summary said. "The victim pulled over, winding down his window, he said, 'What is your problem?'
"The male walked over and said, 'You.' He then punched the victim three times in the head with a closed fist."
Hutt Valley area commander Inspector Mike Hill said it was a crazy attack, targeting a cop.
"As you see in the video, the guy just comes straight over and lays into him. It's a crazy incident to have taken place. (Tofts) knew he was a police officer.
"He was simply never going to get away with assault on a police officer, and certainly not an off-duty officer out with his family. Those sorts of incidents clearly will not be tolerated."
Tofts was formerly known to police as a Satan's Slaves member. His brother, who was previously also a Satan's Slave, had patched over to the Head Hunters. It was possible he was trying to earn his stripes with the Head Hunters, said Police Association president Greg O'Connor.
"This, one can only assume, is about someone ... trying to prove themselves. It was probably about earning a patch."
Officers were often targeted for intimidation by gang members, though they were less often assaulted, he said.
"The gangs ... are out there gathering intelligence on police the same way police are on the gangs. It's not unusual for police officers to have threats made against them." The weak sentence put society in danger. "It's an insult - 110 hours' community service, they'll be laughing."