Only two minutes from the end, he capitalised on a superb play by replacement back Quinn Tupaea, who won a turnover from Springbok fullback Willie le Roux to gain the penalty.
Barrett had been hoping for a penalty chance and realised as Tupaea went to work that the opportunity was coming.
"I was about 30m back in my defensive position at fullback, but I was running forward as soon as I saw Quinn get over the ball – even before the referee put his arm up.
"Ardie [Savea] knew as soon as we won the turnover it was in my range so, I didn't communicate with him, I just went up to the ref and pointed to the sticks.
"I held my breath a little bit, I didn't want it to draw too much more, it just snuck inside the right-hand upright. I felt like I struck it pretty sweetly," he said.
Because he had been kicking the ball so well, he felt confident he could land it.
"Sometimes as a kicker, once you're in a rhythm, you want those kicks," he said.
What made his achievement all the more remarkable was that he was able to complete it, having been on the end of a barrage of high balls throughout the game, something that spoke of his strength and fitness in being able to cope.
Barrett said he had never had so many contestable kicks to cope with in a game.
"It is hard to combat, but it's a challenge I was relishing.
"Every time they set up for a box kick, I knew it was a chance to own the space in the air.
"We've got to find some ways to combat that because they throw all sorts of different scenarios at us. They are doing it from their attacking 22 and putting it on the head of isolated guys with plenty of chasers, so it can be hard to deal with, but this week, we've got plenty of time to come up with a plan," he said.
By kicking to the 22m area on attack, the South Africans tried to create a lottery by crowding the catching zone, hoping to pick off the error ball to score their tries.
"It's hard enough trying to create your own space when those kicks are shorter, so the positioning in the first place is crucial, so you're there early. You also want to generate enough pace and speed to get in the air.
"They do it well. They have one rocket chase that gets in the air and disrupts, and then they've got their big forwards waiting there to smash you as soon as you hit the ground, so it is a challenge," he said.
Wing George Bridge hadn't been so fortunate when dropping an early ball that created the try for wing Sbu Nkosi.
It had been an uncharacteristic mistake. Barrett said Bridge was a player who caught everything at training and had pulled off a near-impossible catch on the sideline only moments later.