Doctors say a New Zealand truck driver who inflated "like a balloon" when he fell buttocks-first onto a compressed air nozzle is lucky to be alive.

Steven McCormack was working on his truck at Opotiki on the North Island when he slipped between the cab and the trailer, dislodging the compressed air hose that feeds the brakes.

The brass fitting to which the hose had been attached pierced Mr McCormack's left buttock in the fall, sending compressed air rushing into his body.

The 48-year-old said he felt as if he was going to explode and began to scream as his neck, feet and hands swelled up.

"I was blowing up like a football ... it felt like I had the bends, like in diving," he said. "I had no choice but just to lay there, blowing up like a balloon."