Police conducting a search left behind confidential information which was picked up by the Mongrel Mob, a report issued by the Independent Police Conduct Authority shows.
The police watchdog says leaving the operation order at the scene amounts to misconduct, but it has not been able to identify which officer was responsible.
The incident relates to Operation Whiteware carried out in Lower Hutt in February last year, after police acted on a complaint from a woman alleging intimidation and burglary.
The operation order identified the woman and who she was living with at the time.
The report's author, Justice Goddard, is recommending that police review their policy on the handling of operation orders.
In a statement on Wednesday, Lower Hutt Police said they accepted all the findings of the IPCA investigation into how the Mongrel Mob obtained a confidential Police operation order.
Changes were brought in last year to address deficiencies around the security of information identified in the report, the statement said.
As a result of the incident, officers are being educated about their responsibilities around the handling of confidential documents.
Gang 'knows who left document'
The Mongrel Mob says it knows who left the document at the house but was never asked that question by the IPCA.
The report was left at Anthony Pairama's home, and he describes the officer as a careless, obnoxious police woman who didn't show a search warrant until long after entering.
Mr Pairama says this ranks as the clumsiest of all the 100 searches at his property over the years.
Police Association president Greg O'Connor, it's a case of human error, not corrupt behaviour.