"Then he pulled over and opened up his door, and I saw three or four of the front people get off their motorbikes and walk over to the ute."
The witness said he didn't see what happened next as he was in a moving car that passed out of the area.
However, a source told the Herald the gang was the Tribesmen.
The source said one of the riders was clipped by the vehicle, but when the vehicle's occupants stopped to check on the rider they were attacked.
It is understood the Tribesmen were returning to Auckland from Taupō where they had been meeting with other members from across the country.
The eyewitness said it hadn't looked like the ute intentionally cut off the riders, but had rather been trying to let them pass.
Another eyewitness told the Herald they saw medics working on a person lying on the side of State Highway 1, with their chest bare and medical equipment connected to it.
The person said the ute at the scene had three large holes punched into its back window.
Police said they had begun investigating an incident "that has left a man in a critical condition".
"Police were called about 4.45pm to the expressway south of Rangiriri following multiple reports of an altercation involving a number of people and vehicles, including a significant number of motorcycles," a police spokeswoman said.
"Initial indications suggest that during the incident a man was pulled from a vehicle and assaulted."
The man has been taken to hospital.
A section of the expressway remains closed while a scene examination is completed, and diversions are in place, police have said.
"Police would like to hear from anyone who witnessed this incident or anyone who might have dashcam footage that could assist our inquiries.
"If you can help, please call 105 and quote event number P049885631.
"You can also call Crime Stoppers on 0800 555 111."
A motorist travelling south on the expressway posted a video that showed long lines of slow-moving northbound vehicles.
