Cabinet signed off tough new measures to tackle gangs on the basis of inaccurate information which over-estimated the scale of the crime problem.
The briefing paper told ministers 4000 gang members alone were responsible for a huge number of drug and violence crimes, including murders.
The release of the document through the Official Information Act followed the earlier discovery the wrong information had been used in a press release to justify the policy.
Cabinet ministers were also given the wrong information.
The information, which appeared to have been provided by police to the minister's office, went before Cabinet in June before the August announcement.