Former sole-charge Owaka officer Senior Constable Murray Hewitson was behind two search warrants executed in December 2020 which netted police a large cannabis haul.
But the Court of Appeal last month ruled his "multiplicity of careless errors" meant the evidence obtained during the raids was inadmissible at trial.
This month, charges against two 42-year-old men were dropped.
Snr Const Hewitson has since left his post in Owaka and is now working in Dunedin.
In 2016, police first received information through the Crimestoppers hotline about one of the defendants, but took no action.
By April 2020, there was another call alleging similar cannabis offending, prompting Snr Const Hewitson to step in.
It was another four months before he obtained power accounts for the rural Owaka property and it was nearing the end of the year when he applied to the court for a search warrant.
Snr Const Hewitson wrote that an increase in electricity usage at the farm was consistent with indoor drug growing.
But there were major flaws in his conclusion.
The officer had obtained no power readings from before the men lived there, so no comparison could be made, and no inquiries were undertaken as to what kind of business they were operating.
The target of the warrant was not even the account holder for the bulk of the period, the Court of Appeal noted.
Also, Snr Const Hewitson did not include in his warrant application the name or address of the informant who had given police the tip-off.
The Court of Appeal said there was simply insufficient information to justify a search warrant.
"The only available inference is one of gross carelessness," Justices Miller, Ellis and van Bohemen said.
"This cannot be minimised as a matter of oversight or poor presentation — there is an inference of indifference to be drawn. The multiplicity of careless errors suggests to us that preparation of the application for the search warrant ‘significantly strayed from the dictates of prudent police action’."
At the Owaka property, police found a "relatively large-scale, professionally built, cannabis grow" complete with insulated panelling, timer-controlled lighting, temperature, air filtration and watering systems.
There were 26 plants in a locked shed and 36 seedlings in tunnel houses outside.
Among those at the home was 41-year-old Henry Smith, who told Snr Const Hewitson he lived at the address alone and tended to the crop, some of which he brought from his previous home in Wanaka.
The officer almost immediately called a justice of the peace to make an oral application — a step permitted if police must act under urgency — to search the Wanaka address.
Neither Snr Const Hewitson nor the JP made notes of what was said during the call.
Afterwards, the police officer drew up his own warrant and sent it to colleagues who found a similarly sophisticated cannabis set-up.
The documentation was neither sighted nor signed by the JP.
When the case came before the Dunedin District Court, Judge David Robinson said the formalities around the process were "sadly lacking".
But he concluded: "There is no evidence that the police acted in a cavalier fashion."
Judge Robinson ultimately ruled Snr Const Hewitson’s impropriety was not at a level where the evidence obtained through the searches should be excluded.
That decision was challenged by counsel Tudor Clee and Allan Tobeck - and the Court of Appeal sided with them.
"Given his seniority and years of experience, it must be assumed that Snr Const Hewitson was aware of the requirements of the Search and Surveillance Act and knew that the warrant he was purporting to execute was non-compliant," it said.
"The right breached in each case [the right to be free from unreasonable search and seizure in the context of private dwellings] is an important one and we would term the seriousness of the intrusions on it as grossly careless."
Smith pleaded guilty to two charges of cultivating cannabis and in 2021 was sentenced to community detention, community work and supervision.
Police were unable to comment on the case before deadline.
