Police bagged up firearm parts after a search of an Auckland property - and then forgot to take them away.

The mistake was denied until the gun dealer at the centre of the search sent photos to the Independent Police Complaints Authority (IPCA), showing the lower rifle receivers that were left in a bag at the rear door of the property.

A receiver is the metal piece that holds the mechanisms that allow guns to fire. Receivers have serial numbers and are strictly controlled.

Police say at no time was there any risk to the public, and the bag containing the parts was left at the rear door for a very short period of time, and while the dealer remained at the property.

However, the author of a gun blog that has tracked the police bungle told the Weekend Herald if the receivers fell into the wrong hands, it wouldn't be difficult to turn them into firing weapons.

There are YouTube tutorials on how to complete guns from receivers. John Zawahri killed five people in a shooting rampage at a Southern California college after obtaining a receiver and building a rifle himself.

"The AR [Armalite Rifle] platform is very flexible and can be assembled to meet a variety of roles by selecting different component parts. But it is useless without a receiver. For this reason that vital component bears the serial number," said Mike Loder, contributor to Kiwi Gun Blog and shooterslottery.org.nz .

"We've seen a culture of casual dishonesty become established in the police responsible for managing the Arms Act. This needs to change and accountability return".

Police officers executed a search warrant on a Howick property on December 18, 2014.

It's understood the firearms dealer had temporarily stored weapons in safes in the house of an acquaintance, which he claimed was done with police approval.

However, when police carried out a search warrant they were concerned at the lack of security for the large number of firearms at the address. Some of the parts were removed by officers and taken to another property where the dealer had an approved storage facility.

It was during this process the bag of receivers was mistakenly left behind at the rear of the property.

Loder was told of the mistake, which he addressed in an October 2017 blog post.

Police told him and others interested in the case to submit questions on the matter through the Official Information Act (OIA) process, which requires government organisations to respond within 20 working days.

Response deadlines were missed and some requests rejected, including because an employment investigation had started after the IPCA became involved. The Office of the Ombudsman contacted police, after complaints about how OIA requests were handled.

An eventual response from police has now been put online. On May 7, Detective Sergeant Mark Osbourne, professional conduct investigator for the police, emailed a colleague to advise he had been to see the firearms dealer, and get the photographs of the firearms parts left behind.

"I needed to cover this off because the IPCA had already interviewed him and got the photos," Osbourne wrote. "There is a bit of debate about whether a lower receiver is a 'firearm'...anyway it's kind of a mute point because we did leave something behind and so as a result I will have to amend the report I did to cover this off."

A police officer involved in the search was spoken to again, and said they "couldn't recall the context of the conversation when the subject of the search had rung to advise him".

Police won't disclose the outcome of a now completed employment investigation. The firearms dealer didn't respond to requests for comment, nor did the IPCA.

Inspector Steve Clark, Auckland City professional conduct manager, said the search was for evidence around the unlawful possession and supply of firearms to unlicensed persons.

"We were not initially aware that firearms parts were left at the address. It was only through further investigation that this information was brought to our attention...the parts left behind weren't capable of firing a projectile, they required a number of other parts to render them useable," Clark said.

Charges were laid as a result of the search warrant, but Clark said he couldn't comment on matters that had progressed through the courts.