The accident happened about 12.30pm, at the fourth obstacle in the fourth event of the challenge.
An eye witness said Mr Jeffery's home-built 4x4 was on a downward run when it rolled forward on to its roof cage and then back over on to its front. That was when the whiplash-like impact happened.
Rolling and spinning are common in 4x4 events, and the crowd initially gave a chorus of approval while the event announcer said he would give the two entrants time to sit in their truck and recover.
The witness said the crash didn't look serious: "I've seen far worse," he said.
Track volunteers and medical staff carefully lifted Kapiti Coast man Mr Jeffery from his truck and carried him up the bank on a stretcher, wearing a neck brace. He had suspected chest and back injuries.
He was taken to Whanganui Hospital, and a spokeswoman later that afternoon said he was in a stable condition.
His co-driver Mark Pincock was unhurt.
The challenge had another eight events, and the rest of the field carried on into the evening.