Police officers who crossed a legal line to pursue gang members are the focus of an internal investigation which could lead to criminal charges.
Police finally conceded they were investigating their own - but only after a second High Court judge released details of the police response to an undercover operation which went out of control.
Charges against 21 gang members were tossed out of the High Court last week after Justice Simon France found officers had committed a "fraud" on the courts during an undercover operation.
The Solicitor-General yesterday filed an appeal against the decision to order a halt to the prosecution of Red Devils members and associates.
Earlier, Justice Forrest Miller revealed the police were conducting an internal investigation in a decision which barred media from unfettered access to the court file. He said the possible prosecution of officers involved in the case meant access to the court file needed to be restricted.
He said officers who had testified, which included a detective superintendent, might challenge evidence previously given in the case "were they to be prosecuted themselves".
"An internal police investigation is being held. The Crown assumes that disciplinary action or even criminal proceedings cannot be excluded at this stage."
The prosecution was halted when evidence emerged of police behaviour during the investigation and subsequent court case. Officers faked a search warrant and invented a signature purporting to be a judicial officer's. They then staged a fake arrest of an undercover officer, swore a false oath to put the case through the court and arranged approval from the Chief District Court Judge, later rewriting the police manual to support their steps.
Justice Simon France said a review of the case showed "a fraud is being committed on the courts".
The operation was run by the Organised and Financial Crime Agency of New Zealand, an elite police group charged with disrupting criminal groups. It has also come under scrutiny for its handling of the Kim Dotcom case.
Justice Miller said the Herald application to access the file was supported by "strong public interest in police actions" which led to the case being tossed out. But he said the court also needed to consider "fair trial rights of the affected officers".
Police had avoided making state-ments about their response to actions revealed during the Red Devils case.
Last night, a police spokesman confirmed the investigation, saying it would "establish any issues of a criminal or disciplinary nature".
"Police are absolutely committed to ensuring the undercover programme operates in a way which puts offenders before the courts while operating within the law."
The spokesman said the investigation ran alongside a review of the undercover programme "to ensure this situation does not happen again".
Police Minister Anne Tolley's off-ice confirmed yesterday she had discussed with Deputy Commissioner Mike Bush the police decision to review "aspects of their undercover operations" and agreed with it, but refused to expand further.
Labour police spokesman Kris Faafoi said an independent inquiry was required. It needed to be outside the force, with a view to maintaining faith in the police investigations.