He has strenuously denied the allegations, including under oath, and is fighting to keep name suppression.
Fangupo and Kulu imported large amounts of methamphetamine into the country between June 2017 and January 2018.
Five packages, containing 14.8kg of meth, were intercepted at the New Zealand border. Two packages were intercepted at the United States border.
Four packages were not intercepted by either country's officials but communications implied they contained several kilograms of methamphetamine.
Fangupo, Kulu, Finau and Halane Ikiua were sentenced for their various roles in the scheme by Justice Mathew Downs last year.
In a judgment released today, the Court of Appeal deemed a lower starting point was warranted than the one used for Fangupo's sentencing.
The young man also did not receive a discount for previous good character at sentencing despite having six references including positive words from his church leaders, former employer and a high school teacher.
He was described by one church leader as being "humble and kind".
While Fangupo was given a 10 per cent discount for good rehabilitation prospects the sentencing judge was not persuaded to give any discount for previous good character.
He observed that such discounts are usually reserved for "older offenders" who had lived an otherwise blameless life.
However, the Court of Appeal has taken a different view: "By the time a young man reaches his mid-20s he is likely to have shown clear indications of his character."
The three presiding appeal judges have deducted 15 per cent from the starting point to reflect Fangupo's prospects of rehabilitation and his previous good character.
During the appeal, Fangupo's defence lawyer Tiffany Cooper had also argued the sentencing judge failed to properly differentiate between the roles of Kulu and her client.
She argued Fangupo was lower in the hierarchy of offenders than Kulu.
But the Court of Appeal judges said all the communications showed was that Fangupo was having difficulty controlling Kulu because of his "cavalier attitude" towards money.
"The conversations do not undermine the proposition that Mr Fangupo and Mr Kulu played 'complementary leading roles' in the enterprise."
The sentencing judge was entitled to conclude there was "little material difference" in their roles, the judgment said.