The judges scored the fight 114-113, 115-112, 115-112 in favour of Barrett who is about to embark on a career as a professional wrestler.
Tua was sluggish and subdued in the opening rounds with Barrett using his reach advantage to keep the former challenger for the WBC, IBF and IBO world heavyweight titles at bay.
Barrett's jab kept a flabby Tua on the periphery and his right hand and left hook combinations ensured he won rounds two and three as Tua looked out of sorts.
He opened up a cut above Tua's right eye in the third round and it was a more active Tua who emerged in the fourth round, but he continued to be out-thought by the 100.9kg American who racked up round after round against his ineffective opponent.
Tua's major weapon, the left hook landed only fleeting blows on the tall Barrett and then failed to follow up some up his rare successes with anything more substantial.
It took until the 10th round before Tua, who weight in at 110.8kg before the fight, sparked into life and even then, despite landing two left hooks in quick succession which wobbled a tiring Barrett he could not follow up with anything of substance. The American was able to regather his senses and avoid the stalking Tua by moving side to side and staying outside his shorter opponent's reach.
With Tua needing a knockout to snatch the victory he upped the ante in the closing rounds and despite flooring Barrett with under a minute left to go in the round with a vicious left hook followed by two powerful rights, could not land the killer blow to save the fight and possibly his career.
On the undercard, promising amateur heavyweight Joseph Parker claimed a points win over Australia's Nathan McKay.
Light middleweight Steve Heremia forced Brad Pohl's corner to throw in the towel after a crushing right hand in the second round of their professional bout, while super middleweight Isaac Peach had a unanimous decision over Gunnar Jackson in their six-round fight.
- NZPA