He hit it straight and thought the wind would drag it away from the green.
"I've been pin high before – I can hit the driver well but I'm usually left or right," he said.
"I couldn't believe it. One of my mates found the ball in the hole. They were dumbfounded as well."
Duff normally plays with the popular Titleist or Srixon balls but was using an uncommon brand his wife Andrea had put in his Christmas Santa sack.
So when he saw it staring back at him from the hole, it helped him get over the sense of disbelief because he knew it had to be his ball.
"Those cheap balls are usually like hitting concrete but these were actually quite good," he said.
Duff, a 10-handicapper, launched into a celebration dance which drew the attention of fellow club members who raced over.
They quickly advised him to put the ball away and the Waiuku club will have it mounted for him.
Duff said members were "generous" in helping him celebrate which meant Duff needed a lift home.
"I was pretty dusty this morning," he said.
The remarkable ace made up for an early triple bogey, and he finished with a round of 77.
"I didn't really care about the game after that – I just wanted to finish so the hole in one was official," said Papakura-raised Duff, who was 12 when he first started playing golf with his dad.
"I'll never reach that pinnacle again."