The injured man, who was flown to Dunedin by the Otago Regional Rescue Helicopter, was the sole occupant of his vehicle.
Two Ranfurly men in the other vehicle escaped without injury.
Senior Sergeant Mike Cook said it appeared one of the vehicles had failed to give way.
The two occupants of the south-bound vehicle had walked away from the crash as they were wearing seat belts.
One of the occupants told police he had received a ticket for not wearing a seat belt a few days before, and he realised his life had probably been saved because he had buckled up.
"People get upset when they get issued a ticket for not wearing a seat belt but this is one of many cases that proves belting up is a serious thing, and in this case it most certainly saved the occupants of the vehicle from serious injuries or even death," Snr Sgt Cook said.
The twin cab ute was towing a trailer with a small diesel tank on it and another diesel tank was on the deck of the ute.
In the accident, the two tanks broke free and landed several metres down the road.
