Since 2005, the weight of the Class A drug has been the most important factor in sentencing individuals convicted of manufacturing, importing or supplying methamphetamine.
Depending of the quantity of methamphetamine, the guideline judgment established four bands – ranges with an upper and lower limit - of prison sentences.
Anything over 500g of methamphetamine is considered to be in the most serious Fatu band, with a minimum starting point of 10 years in prison and a maximum of life.
Only once someone is placed into one of the four bands is an individual's role - whether a lowly drug "mule" or kingpin at the top of the criminal organisation - taken into account by the judge.
These personal circumstances can move a defendant with the upper and lower limits of the band, but not between bands.
But the quantity of drugs, argued Corlett, should not be the most significant marker of criminal culpability.
He pointed to one of his clients, a 20-year-old from Hong Kong who was paid $10,000 to supervise the delivery of 60kg of the drug.
Lok Sing Yip was sentenced to 16 years, six months in prison, despite the judge describing him as a "mere employee" of an organised crime syndicate.
"Deterrence [from long prison sentences] is not in fact something that changes behaviour, or taken into account for decision making, except for those at top of the organisation," said Corlett.
Lowly "catchers" who receive the drug packages, or "mules" who physically smuggle the drugs, were disposable to the organised criminal organisations who took advantage of them, said Corlett.
He favoured a more nuanced approach where quantity and role were taken into account when determining someone's culpability.
It was a submission echoed by Andrew Butler, on behalf of the Human Rights Commission, and James Rapley QC, on behalf of the New Zealand Law Society and New Zealand Bar Association.
The Law Society submitted Fatu's focus on the quantity of methamphetamine, and the need for long sentences as a deterrent, had created a "rigid" approach to sentencing.
This restricted the discretion of the sentencing judge, which was at odds with the other principles of the Sentencing Act.
"Quantity should not be a proxy for culpability," said Rapley.
He pointed to the evidence of police, who said gang members were giving small samples of methamphetamine to solo mothers, for free, to get them addicted. The women would then become dealers to the gang to pay for their addiction.
"That's a small amount of methamphetamine, but the culpability [of the gang members] is great. It's causing significant harm to the community."
The quantity of methamphetamine should be one of many factors taken into consideration, said the Law Society.
These included the role of the offender, addiction, gender, poverty, cultural factors, links to organised crime, how much money someone was paid, as well as the exploitation of others.
A rare "full bench" of five Court of Appeal judges, headed by president Justice Kos, will hear the submissions of the Crown in response to the appeal on Wednesday.
