Mr Duffy reached the destroyed car first and saw the driver in the passenger footwell.
He checked for the driver's vitals and realised he was dead, so went to help another man working to free the occupants of the SUV.
Smoke was already billowing from the vehicle and flames were creeping out of the bonnet.
Two people had been pulled free but two young girls were stuck inside, Mr Duffy said. "I was trying to reach in and help them ... grab a knife, cut the seatbelts. It was all crumpled up so we couldn't really get them out.
"I looked up and I saw the flames starting to get quite big and I thought if I don't try to subdue that somehow then we're not going to get them out."
Mr Duffy yelled for fire extinguishers from any of the nearby vehicles and five people came forward with them. It was just enough to keep the blaze under control before the two girls were cut free and dragged to safety.
But there was more work to be done - banging from the trailer alerted the rescuers to a horse that was trapped inside.
Mr Duffy broke the metal bar restraining the horse, which then bolted outside and immediately put its head down to graze, he said. The whole rescue took 10 minutes.
Mr Duffy said he was the right man to be first on the scene: he is a volunteer firefighter in Kinloch and was in the police for 12 years before becoming a truck driver seven years ago.
"With my background and training, I didn't really think about it until after, you know. It's just doing my job. I wouldn't say I was a hero, just in the right place at the right time, I guess," he said.
"Everyone did their part. I've seen that scene a few times. But it's not often that it literally happens right in front of you. You're doing your job and driving and then all of a sudden the whole thing just changes in seconds."
Tracey Doyle, who lives on a farm near the crash, said she heard the vehicle explode.
It was an extremely close call for the passengers and the horse, she said.
"There was a big explosion we heard and it went up in smoke. If they hadn't pulled them out I would say there would be a different story today. They are heroes," she said.
Acting Inspector Nigel Allan said preliminary investigations suggested the car driver, Kent James Bond, had overtaken a number of vehicles in the passing lane before losing control.
Excessive speed was indicated to be a factor, he said.
Mr Allan said the men who pulled the passengers free almost certainly saved their lives.
"While this is an absolutely tragic crash, I have no doubt the selfless and heroic acts of those persons who came to the assistance of the people trapped in the burning vehicle have saved four lives."
The driver of the SUV, a 41-year-old, was in a stable condition in Palmerston North Hospital's intensive care unit this morning and the three children have all been discharged.
