Hundreds of illegal firearms are being seized each year in police raids - but there is no record of how many guns are stolen to fuel the black market.
New figures show 325 firearms were seized in the financial year to June, the lowest number in the past five years and down from a peak of 1146 the previous year.
Most of the illegal firearms are stolen in residential burglaries or from collectors by organised criminal groups, police documents released under the Official Information Act show.
"Research indicates there is already a large pool of illegally held firearms in New Zealand and that firearms of almost any type can be obtained relatively easily from within the criminal fraternity without needing to source illicit firearms from overseas," according to the 2011 National Strategic Assessment paper.
But a request for the total number of guns reported stolen in each police district over the past years was refused - because the records do not exist.
"Reports of stolen firearms are received in districts in hard copy form and retained in districts. That information is not collated or analysed centrally," Superintendent Barry Taylor wrote in response.
"Police do not therefore hold the total number of firearms reported stolen."
Critics say firearm record-keeping in New Zealand is poor. The system of licensing each firearm was abandoned in 1982 in favour of registering gun owners instead.
The result is the police have no accurate idea of how many guns - legal or illegal - there are in the country or how many stolen guns are in circulation.
Estimates from Sir Thomas Thorp's 1997 Review of Firearms Control in New Zealand are still quoted: 100,000 held by unlicensed owners of a total of 700,000 to 1 million.
"In the nature of things it is impossible to get an accurate count of the number of guns held for criminal purposes," Sir Thomas wrote. "However, there is evidence of a substantial pool ... estimated at between 10,000 and 25,000."
The Thorp report recommended that all firearms be registered - not just handguns, machine guns, automatics and military style semi-automatics - which has been ignored by Parliament.
New Zealand gun control advocate Philip Alpers, now an associate professor at the University of Sydney, says that restricted weapons make up 5 per cent of the national stockpile and are used in just 3 per cent of crime involving firearms.
New Zealand and the United States are among the few developed countries which do not register all weapons, said Professor Alpers.
Weapon registration makes an owner accountable for each firearm in their possession and is a stronger incentive to lock up guns, said Professor Alpers, as well as making it easier for police to trace guns used in crime.
A black market exists in which a gun that a licensed firearms owner could legitimately buy for $3000 can be sold illegally for $12,000.
The most notorious example is the case of John Noel Mabey who claimed his entire collection of 121 restricted firearms (pistols, machine guns and military-style semi-automatics) was stolen from his Coromandel home.
Mabey was jailed after admitting he faked the burglary and had, in fact, sold the weapons to criminals.
Most of his guns have not been found.
Violent crime involving guns is rare in New Zealand and makes up 1 per cent of total violent crime, according to the National Strategic Assessment.
The use of firearms against members of the public was rare but still posed a risk to police staff and the community, the report concluded.