Police say they are "reviewing systems and processes" but have no idea how a restricted document was left in the hands of the Mongrel Mob.
The slip up was revealed a day after Housing New Zealand was found to have left documents with information including the address of a senior staff member and a former neighbour who had been forced to relocate after threats and intimidation.
The HNZ staff member had lived nearby and was quickly moved from her Lower Hutt address under police protection.
A police manual with details on "Operation Whiteware" -- the investigation into the intimidation case and eviction of 10 members of Anthony Pairama's family from five state houses in Farmer Cres, Pomare -- was left on a table at a gang member's house during a raid on February 11.
Labelled 'restricted', the manual contained details of the woman whose original complaint of intimidation sparked the police investigation. It also showed the names and responsibilities of 50 officers working on the case, a risk assessment of the street, details of how raids would be carried out and the radio call signs police used to identify themselves.
Nine Mongrel Mob members and associates were arrested in the raids and a tenth person summonsed. They face a variety of charges including intimidation, burglary, receiving stolen property and possession of cannabis.
Inspector Geoff Gwyn of Lower Hutt Police said any breach of procedure around the security of documents was unacceptable .
"At this stage we don't know for sure how the document got out but it's unfortunate that it has.
"It's important to note however that none of the information in the document compromises the safety or security of any individual. There is nothing new or sensitive in the document that was not already known to the complainant or the gang members or associates targeted in this operation."
Five tenants with gang affiliations in the suburb of Pomare were this month served 90-day eviction notices following allegations of intimidation.
Mr Pairama told TV3 he planned to use the document "in a complaint so we can try and stop the 90-day eviction, or something".
HNZ chief executive Lesley McTurk said evicted tenants would be assisted in finding new accommodation but that would not be in HNZ houses.