Earlier Tua soaked up some punishment from Barrett, who started the fight with more energy and accuracy, before he gradually started to find his range.
It was hard to criticise Tua's courage last night, as he dug himself out of an early hole, but perhaps it was his conditioning that failed him. The work rate wasn't quite there against the agile Barrett and in the end that proved costly.
Barrett looked the more composed throughout the fight, though he also made judicious use of his elbows and the ropes to get himself out of trouble when Tua threatened. The 40-year-old Barrett also knew that he was one punch from the end.
Barrett, in the blue corner last night, was regarded as a journeyman in some circles before that career-defining knockdown of Tua in July last year. He believed he won that fight - a view confirmed by Tua on Friday when he said he felt "in his heart" he lost the bout.
Barrett is underrated. He is a good mover with a solid punch who came into this fight in great shape.
But we were back in South Auckland - and this was definitely David Tua territory. King Kapisi warmed up the crowd, while up in his dressing room an intense looking Tua listened to Carlos Santana. He looked like he meant business as he entered the ring.
He was accompanied by his usual huge entourage of almost 20 people and seemed to put an incredible amount of energy into the Samoan and New Zealand national anthems.
It was a cautious opening, with Barrett first to find his range with the jab, and landing a strong right hand late in the first round. He landed another right flush on Tua's chin midway through the second. Tua was content to soak it up, but was struggling to make any inroads either inside or outside.
A spectator behind wondered: "Why isn't he fighting?" But the hometown man was doing as much as the American would allow him.
To the crowd's dismay, Tua soaked up more punishment at the start of round three and incurred a cut above his eye. His timing seemed off; but this was the time to dig deep.
The fourth round was more even, with Tua finally landing a couple of hooks that rolled across Barrett's chin. He ended the round with a wry smile; perhaps of frustration or perhaps knowing he had soaked up everything Barrett had to give. He drew blood in the fifth round, catching the New Yorker above the eye but failed to land any follow up punches.
Tua was convincing in the sixth round, with a booming left that dislodged Barrett's mouthguard, but again couldn't follow it up. He was starting to crack the safe - but couldn't find the right combination.
At this stage, Barrett looked sturdy and balanced. It was enough to see him home.
