He was caught in a police sting targeting a "large and sophisticated commercial cannabis cultivation operation run by groups with the sole intention of supplying cannabis for profit," according to Justice Downs' appeal decision.
Houses were bought or rented by the group and converted to provide large separate rooms for the cyclical growing of cannabis, and frequently, as in Cao's case, the electricity supply was diverted to avoid authorities detecting the large electricity bills.
A police search of Cao's Pukekohe property found 180 mature cannabis plants in four rooms with a monthly harvest worth an estimated $30,000 a month.
The floors were lined with thick polyethylene, the walls lined with blackout fabric, and fluro lights and carbon filters built into the room with associated ducting.
The bathrooms were converted to supply water-based fertiliser to each of the grow rooms.
Electricity supply was commercially diverted with high-amp fuses, the doors double deadlocked, and security cameras were installed.
Cao claimed he did not know growing cannabis was illegal in New Zealand.
He arrived in 2019 as a tourist, was "trapped" in the country because of the outbreak of Covid-19 in 2020, and grew weed for profit as well as his own use.
He claimed he was not involved in the skilled setup of the growing operation, a point dismissed by District Court Judge Chris McGuire at Cao's sentencing in May.
The judge said Cao's claim he grew cannabis to survive being marooned in New Zealand and was just tending plants "flies in the face of the sophistication of this operation".
There was no information who, if not Cao, was behind the operation, McGuire said.
Cao appealed against his jail term at the High Court, arguing through his lawyer Callum Fredric that it was "manifestly excessive".
Fredric compared Cao's case with that of Hai Hong Pham, another Vietnamese national who was sentenced around the same time to five months home detention for growing cannabis in similar circumstances.
Like Cao, Pham was growing cannabis in a house converted for the purpose, including redirecting the mains electricity supply to run heat lamps, charcoal filtration systems and a sophisticated irrigation network for some 200 cannabis plants.
"The dramatically different result for these two people [with broadly similar offending] is not consistent with the appearance of justice," Fredric said, raising the point of disparity, a principle engaged when two or more co-offenders receive unjustifiably different sentences.
In a decision released last week, Justice Downs said Cao and Pham were not co-offenders so the principle of disparity did not apply.
Cao's sentence was "comfortably within range" of comparable cases, he said.
Counties senior prosecutor for police Paul Watkins said this was a case where criminals were converting good homes into intensive cannabis growing operations in the midst of a housing crisis.
"Some Kiwi families are struggling to pay the winter bill, yet these offenders are diverting power straight from the national grid to power their growing equipment," he told Open Justice.
"This offending should be met with the appropriate sanction from the courts," Watkins said.
