Pat Cashman, who works for the Irish Golf Union, was asked by the triple major winner to step in on the sixth hole of the second round at the Durban Country Club when his long-standing bagman was taken ill.
Ronan Flood, who is also Harrington's brother-in-law, had to leave the course because of food poisoning and the player plucked Cashman out of the gallery to take his place.
"I was following Padraig who I've known for a long time," the 65-year-old photographer told reporters. "Padraig asked if I'd take over the bag but I was very worried at first, about Ronan's health and the weight of the bag.
"The funny thing was that I took over on the sixth and at the eighth he hit a double bogey. I kept apologising after that but in the end it worked out well - he's a lovely chap."
Harrington had nothing but praise for his replacement caddie after carding a second successive one-under-par 71.
"He started off like he was carrying bows and arrows but by the end he had the double-strap going (on the bag) and was using all the lingo," said the former Ryder Cup stalwart.
"It's amazing how much caddies do for you and it gives you a new appreciation for what they do out there. I know now why we pay them the big bucks."
Cashman said Harrington might have done a lot better with his regular caddie.
"I think Padraig could have been five or six under with Ronan on the bag," he added. "I think I'd be a very poor caddie but I enjoyed it.
"I was glad to be helping out. I'm led to believe Ronan is now OK but I've told Padraig I'm on standby if he needs me."
Harrington is five strokes off the lead at the halfway stage of the first European Tour event of the year.
(Reporting by Mark Gleeson, editing by Tony Jimenez; mark.gleeson@thomsonreuters.com +27828257807 Messaging mark.gleeson.thomsonreuters.com@reuters.net) nL3N0KK4M5
