"I only had to carry him 20m to get to bed.
"It was raining and slippery and I just went off the path to avoid getting wet. It was just one of those things."
He was not wearing shoes and lost his footing in the darkness.
Today, from Southland's Kew Hospital, Mr Tetley-Jones praised the efforts of the hut warden and helicopter crew who came to the aid of his family.
He said wife Diane and the remaining three children planned to walk out of the bush, helped by other experienced trampers walking the route.
His son was expected to have a cast put on his leg this afternoon.
With family plans to return to their home in Otorohanga in disarray, Mr Tetley-Jones said both his pride and heart had taken a hammering.
The four day Milford Track hike had been something of a family challenge that only hours before the accident had seen them on a high after successfully scaling the tough Mackinnon Pass.
"I feel like I've ruined our holiday - well the final part at least," he said.
Despite the injury his son was already talking about the possibility of taking a flight into Dumpling Hut once his leg had healed and walking out just to say he had completed the walk.
But Mr Tetley-Jones said one thing was for sure on any return trip south. " I certainly won't be giving him a piggyback from the kitchen to the bunk room ever again."