Finau also pleaded guilty to two charges of unlawful possession of a firearm and unlawful possession of ammunition.
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Today the court heard Fangupo and Kulu, equals at top of the operation, were motivated by profit.
"You were the leading figures in this operation. In short, this was yours," Justice Downs said.
Fangupo had hoped to make millions, he said.
"The Crown alleged you had someone inside our postal services, perhaps even Customs, helping you," Justice Downs said.
"This because your conversations frequently referred to having someone on the inside."
The pair argued there was inadequate evidence of this for it to feature in sentencing.
"I give you both the benefit of the doubt because the topic was not greatly explored in your communications, or at trial more generally," Justice Downs said.
Between June 2017 and January 2018, Fangupo and Kulu imported large amounts of methamphetamine into the country.
"Your offending was reasonably sophisticated. You followed a pattern. You sourced
methamphetamine in California, albeit the drug likely came from Mexico," Justice Downs said.
All of the methamphetamine came to Auckland by post, he said.
Five packages, containing 14.8 kilograms of methamphetamine, were caught at the New Zealand border.
Two packages were caught at the United States border.
"These contained at least 4.1 kilograms of methamphetamine. I say 'at least' because United States officials did not weigh all their contents," Justice Downs said.
Four packages were not caught by either country's officials but communications implied they contained several kilograms of methamphetamine.
Fangupo's lawyer, Ron Mansfield, said the 26-year-old had no relevant previous convictions and that this would be his first sentence of imprisonment, which was significant to the young man.
Mansfield said Fangupo was previously involved in a small clothing label, which presented in a rather humorous way his pride in his Tongan heritage.
"He is artistic, he has talent."
Mansfield argued that someone as motivated to reintegrate into society as his client should not be sentenced to such a lengthy term that it would be institutionalising.
"I do ask there be an end in sight for him."
Kulu's lawyer Steve Bonnar QC said at some point with lengthy imprisonment sentences, the deterrent effect was served and further increases became academic.
"... Longer and longer sentences do not achieve the purpose in which the Crown thinks they do."
Bonnar said Kulu was a young man who once held a promising career in the United States.
Kulu had played American football for Oklahoma University but acquired a shoulder injury and became addicted to opioids - sparking a downwards spiral, Bonnar said.
Justice Downs sentenced Kulu to 18 years' imprisonment, and Fangupo 17 years' imprisonment - both were to serve a minimum non-parole period of 50 per cent of their sentence.
The two lesser characters in the scheme, Finau and Ikiua, were sentenced to shorter terms.
Finau's defence lawyer, Maria Pecotic, said the relatively young man was under significant family obligations and, having no money, did a "very desperate thing".
"He did not appreciate the gravity of the situation he found himself in," Pecotic said.
The 28-year-old was very naïve and gullible, she said.
"He is trying, as your Honour can see, to make the best of the situation he now finds himself in."
He was a good prospect for rehabilitation, she said.
Ikiua's defence lawyer, Belinda Sellars QC, said the 28-year-old had experienced a "fall from grace".
She explained the "extremely hardworking" man would do anything for his family and having started a new business had been under financial strain.
He never actually received any money, Sellars said.
The court heard Ikiua, who had pleaded guilty to one charge of importing methamphetamine before the others went to trial, had excellent references and posed a low risk of re-offending.
Finau was sentenced to eight and half years' imprisonment and Ikiua three years and four months' imprisonment.
Meanwhile, a fifth man, Shane Singh, has already been jailed for 10 years and four months' for importing a Class A drug.