A lawyer for gang members, who had charges thrown out after police brought a fake prosecution against an undercover officer posing as a bikie, hopes the force never tries it again.
Police used a fake warrant to search the undercover officer's lock-up, and then laid false charges against the officer in an attempt to give him more credibility with the motorcycle club he was infiltrating.
Justice Simon France issued a stay of proceedings on the drugs and criminal group charges against 21 members of the Red Devils Motorcycle Club, labelling police actions a "serious misuse of the court's processes".
They thought they had got permission from then-Chief District Court Judge Russell Johnston to lay the charges, but Justice France says a letter given to him was "wholly inadequate" to alert him of the situation.
Lawyer Steven Rollo, who represented seven of the 21, said the police actions were unacceptable.
"The undercover programme has been running in New Zealand for over 30 years. Never before have they employed these sorts of tactics," he told Radio New Zealand.
"Let's hope we never see the likes of it again."
He said the charges against the Red Devils members were not serious, calling them "social offending within their own small circles, by and large".
"I don't think the people of Nelson have anything to worry about these people, bearing in mind they've lived happily in Nelson, many of them all of their lives without incident prior to this."
Police wouldn't comment on the ruling, issuing a one-line statement saying they were studying the judgment and will be taking advice and considering options.
But Police Association president Greg O'Connor says the public are the losers from a decision he labelled as "unjustified and disproportionate".
"They are really punishing the public, who will now see 21 individuals escape trial for alleged serious organised criminal offending, and be left free to carry on their activities."
Police Minister Anne Tolley wouldn't comment on the ruling but said police were doing their best to keep the public safe.
"I'm just disappointed that gang members have not been brought to justice."