"In a situation like this we normally would have been [worried about spinal injuries] but in the seriousness of the situation the doctor who was there said they needed him out now," Mr Fuller said.
"There wasn't a lot of blood but it looked like he might have had a broken femur, which can be a fatal injury because you're bleeding into yourself."
He was taken to Auckland Hospital in a critical condition, but a hospital spokeswoman said today this had improved to stable.
The passenger in the campervan, understood to be the man's wife, and the occupants of the 4WD, a man and a young child, were uninjured.
Mr Fuller said it was unclear whether the trailer had jack-knifed or if it had actually become detached from the 4WD.
"There is speculation that it started to wobble and he applied the brake and it jack-knifed and flung around."
It was no longer attached when emergency services arrived, and had spun around 180 degrees.
The police serious crash unit is investigating.
Mr Fuller praised members of the public for helping to control traffic, and emergency services, who acted "admirably".
The crash closed the Hibiscus Coast Highway for over an hour.
It came just after police issued a caution urging drivers to take care on roads following light rain.
Assistant commissioner road policing Dave Cliff said after such a long dry period, the build-up of oily residue on road surfaces will create a slippery film when it starts to rain.
"Roads will be extremely slippery for some time and drivers will need to be very careful about speed, following distances, and distances it takes to stop."
Mr Cliff said it was also extremely important that people check tyre pressure and tyre tread depth.
"We're asking everyone to take extra care on the roads when the rain does arrive," he said.
