A judge has thrown out a Crown case against 21 accused because police fooled a court into prosecuting an undercover officer.

Justice France has ordered a stay of proceedings in prosecutions of those arrested as part of the police's Operation Explorer. It was a police crackdown on motorcycle gang members.

During the investigation, a fake search warrant was prepared.

Justice France said a search warrant could be issued only by a judicial officer.

''This fake warrant, unappealingly described to me by the officers involved as "a prop", purported to be signed by a judicial officer,'' he said.

''The police scrawled an apparent signature ... asserting the warrant has been issued by a deputy registrar, name indecipherable.''

An undercover officer using the name Michael Wiremu Wilson infiltrated the Red Devils in Nelson and police orchestrated a false arrest to boost his criminal credentials.

"However one looks at it, a fraud is being committed on the courts," Justice France said about the false arrest.

He said it appeared the police's conduct had components of committing criminal offences.

''The search warrant would seem to engage section 256 of the Crimes Acts 1961 and the swearing of a false information would seems to engage section 110 of the Crimes Act.''

He said he thought it reasonable to proceed on the basis that no charges would be laid from the police's actions.

Lawyers for the defendants argued the police's actions were an abuse of the court process and said the charges, which include drugs, firearms and conspiracy to take part in organised group offences, should be thrown out.

Justice France today agreed it was an abuse of court process.

He said: ''I see the actions of the police in this case as involving serious misuse of the court, and a troubling misunderstanding of its functions.

''Anything other than a significant response runs the risk of being seen as rhetoric. In the end I consider it comes down to how serious one sees the conduct as being, and what price the system is being asked to pay in order to preserve its integrity.''

Justice France said it was expected police would act because of the ruling.

''I doubt a false information about a fictional offence will again be sworn,'' he said.

Two police officers told the High Court in Nelson that they had followed police protocols in approaching then chief district court judge (the late) Russell Johnson to ask his permission.

However, the court later learnt the manual had been written after police had pursued the false prosecution and recalled the officers to testify again.

The previous manual, existing at the time the officer approached the judge,  said: ''The police must not allow an arrested agent to appear under a fictitious name without the permission of the court. Deceiving the court is not permitted.''

Justice France said he accepted Detective Superintendent Rod Drew had not meant to mislead the court, but it was ''unwise'' not to seek legal advice about the fake prosecution.

He said a letter presented to the judge was ''wholly inadequate to alert the chief judge to the realities of what was involved''.

In the High Court ruling, Justice France said police believed the ''emerging prominence'' of the Red Devils was a  forerunner to it becoming a chapter of the Hells Angels.

Officers obtained warrants to intercept telephones, text messages and to install listening devices. They also infiltrated the club, with two undercover officers posing as a couple.

However, there was always ''a level of suspicion'' among club leaders about "Wilson", the judgment said.

His handlers were concerned he would be exposed and devised a strategy to ''strengthen his credibility''.

They obtained the fake search warrant for a storage lockup rented in "Wilson's" name. Police wrongly believed the owner was involved with the club.

Police placed in the lockup some ''apparently stolen'' equipment, as well as items associated with cannabis.

''On its face, the warrant appears genuine,'' Justice France said.

Police then called in the lockup owner and showed him the warrant, and then the contents.

"Wilson's" handlers then called a meeting and decided to go ahead with the ruse by pursuing a prosecution.

"Wilson" was charged with possessing equipment to cultivate cannabis in May 2010.

A police officer, known as Constable X, swore a ''false oath'' to the court.

''MW had not committed such an offence, and Constable X did not suspect he had,'' the judge said.

Police expected "Wilson" to plead guilty ''reasonably quickly'', but on advice from gang members he decided to get a lawyer and the process became protracted.

Police decided to go along with this to boost "Wilson's" standing  and that if he failed to appear before the court it would boost his credibility.

Two warrants had to be issued and a further charge of breaching bail was added.

The owner of the storage facility ''has certainly been the victim of improper police conduct,'' the judge said.

And the ''the court's processes can truly be said to have been abused, first by the use of the warrant, and second, by the laying of a false charge''.

He said judges had been treated ''in a disrespectful way''.

A prosecutor and the defence lawyer were also misled.

He also concluded it was ''a fundamental and serious abuse of the court's processes''.

Justice France said: ''The courts are not part of police investigation. There is and can be no suggestion of collaboration. The court is independent and sworn to treat all who come before it equally and without favour.''

He was ''surprised'' by the lack of insight by the officers ''about the lack of propriety involved''.

Operation Explorer was headed by Detective Inspector Grant Wormald, who also supervised the joint FBI-police raid on Kim Dotcom's Coatesville home for the Organised and Financial Crime Agency.

- © Fairfax NZ News