It was only in prison Magon had realised she had an addiction issue and she was undertaking an intensive drug and rehabilitation programme within the prison system, Justice Nation said.
He summarised the facts as read in the Alexandra District Court in October.
In June, Central Otago police became aware of cannabis use by high school pupils aged as young as 13 and an investigation established Magon was dealing to teens aged between 13 and 17.
The deals were generally coordinated by a 14-year-old girl who would contact Magon by text.
On other occasions members of the peer group would directly or indirectly contact Magon.
The cannabis was sold in $50 bags, each containing about 2g.
Texts and bank records showed Magon sold cannabis from February to the time of her arrest.
Direct payments to Magon's bank account showed she sold cannabis to under-18s at least 19 times between February and June and regarding supply to over-18s, texts and bank records showed she sold cannabis from July 1, 2019, to the time of her arrest at least 64 times.
On June 24, Magon returned to Alexandra from Canterbury in her car alone.
She arrived at a flat inhabited by teens and police followed her with a search warrant - in her car officers found 450g of cannabis broken down into 16 bags.
A search of Magon's address uncovered a "tick" list - featuring many of the teens she had been identified as dealing to and there were hundreds of unused snap-lock bags.
Justice Nation said he was satisfied there was no error in the way Judge Turner had arrived at his sentence.
He commended Magon for the efforts she was making, but said her sentence of imprisonment would allow her to continue those efforts before dismissing the appeal.