Two men laughed as they smacked a truck driver through his cab window after he cut them off on the Southern Motorway yesterday.

Several children and a woman nursing a baby sat in their crimson Mitsubishi Chariot, which was parked in the middle of the motorway, with the woman egging the attackers on.

Auckland motorways manager Inspector Dave Walker said the truck driver had not come forward and police were looking for the attackers.

Witnesses told his staff the northbound truck driver made a lane change near the East Tamaki Rd on-ramp and cut off another car at midday.

Mr Walker said it appeared the assault was "deliberately done as a counter measure" to the lane change.

"Police have some details from witnesses who didn't see everything but saw the driving that led up to that."

White Waste rubbish removal driver Anthony Purchase was driving about 100m behind and saw the Mitsubishi pull up. "It stops in the middle of the motorway and the driver and passenger get out," he said. "They were solid guys."

Mr Purchase pulled his truck over, as did a couple of others, and put his hazard lights on, to stop traffic behind.

"It was potentially a major hazard because traffic can't just stop like that."

But traffic halted for several minutes with no trouble. Mr Purchase was horrified to see the children and nursing woman in the car.

"The woman was acting proud of them," he said. "I just saw them jumping up taking swings at him, both at the door and obviously having to jump up to hit him."

The truck driver did the right thing by staying in his cab, where he had the height advantage, he said.

Mr Purchase said the men then walked back to the car, smiling, and took off. He did not get a look at the registration plate. Mr Purchase said he looked over at the truck driver - a "big burly guy in his 30s who looked as if he could look after himself" - to check he was okay.

"He raised his finger as if to say, 'I'm sorting this out'."

The truckie was on the phone and Mr Purchase assumed he was notifying police. But they have not heard from the driver and Mr Walker urged anyone with information to contact him on 027-235-4562.