Gaping holes in gun laws mean the chances of police discovering a firearms owner breaking the law was "negligible", according to a briefing to Police Minister Stuart Nash.
One example was an A-category licence holder legally purchasing a semi-automatic and converting the weapon into a Military Style Semi Automatic (MSSA) by slotting in a high capacity magazine - exactly what the alleged Christchurch terrorist did.
It's illegal to do so but the sale of high capacity magazines - some of which can hold 100 rounds - is unregulated.
The November 2018 police briefing, obtained under the Official Information Act, was responding to questions from Police Minister Stuart Nash who was considering amendments to the Arms Act.
Nash queried whether police were putting enough resources into enforcement of current obligations of licence holders.
"It is not solely a question of resourcing, though the gap between revenue from the licensing and permitting regime and the costs of managing that regime is large and growing," the report said.
"Beyond resource limitations, there are gaps in the legislative system that make the system difficult or unable to be enforced."
As well as pointing out the lack of regulation on MSSA parts, such as high capacity magazines, police noted several other loopholes.
While it is an offence to supply "A category" firearms to anyone without a licence, there is no requirement on non-dealers (the majority of firearms licence holders) to maintain a record of the person to whom they sold the firearm.
There is also no system for dealers or non-dealers to confirm the licence status.
"Those supplying firearms may not necessarily recognise a fraudulent licence or may be prepared to accept the advice of the purchaser rather than insist of viewing the licence."
Likewise, there is no requirement to record sales of ammunition.
And while A category licence holders have to meet the safety storage conditions, there is no provision for police to inspect the storage to ensure the conditions are being met.
"No risk management system is perfect but, should an 'A' category licence holder choose not to comply with the intent of the [Arms] Act the probability of this being discovered is negligible.
"With 250,000 firearms licence holders, even marginal levels of non-compliance can lead to a significant number of firearms ending up in the wrong hands."
In the wake of the 50 people shot and killed in Christchurch, Prime Minister Jacinda Ardern spoke of urgent reform of gun laws.
"As soon as New Zealanders hear this person was legally able to acquire those weapons and carry out this event, that will raise enormous questions with our gun laws," Ardern said.
"And that is why we will respond swiftly."
An announcement on changes to gun laws could be announced as soon as today following a meeting of Cabinet.
The issues covered in the police briefing to Nash have been raised before, most recently in a 2017 Parliamentary inquiry which investigated how firearms were being illegally obtained by criminals.
Noting concerns about "A category" firearms being easily converted to MSSA with large capacity magazines, the report recommended police investigate a new semi-automatic firearm licence category.
This would include rifles and shotguns and replace the MSSA "E category" licence.
There is also no national register of individual firearms, so the police do not know how many firearms are owned by each licence holder.
However, the police do know how many pistols, restricted weapons and MSSAs are possessed by licence holders.
This is because such weapons can only be supplied to a person holding a "permit to procure".
The parliamentary inquiry recommended extending the "permit to procure" to all sales of firearms, which would allow police to slowly build a database of firearms owned by individuals.
"Although there would be an administrative burden on buyers, sellers, and the Police, the process would ensure better monitoring of private sales."
Both recommendations were rejected by the then Police Minister Paula Bennett following strong opposition from pro-gun lobbyists.
Following the Prime Minister's comments on Saturday, the Police Association - which has long advocated for tighter gun control - said there was no place in the upcoming debate for the radical gun lobby.
"The bitter irony with this alleged perpetrator in Christchurch is he would not have been able to buy the weapons he had in his home country of Australia," said Chris Cahill, president of the Police Association.
"Immediately after the Port Arthur massacre in 1996 the then Prime Minister John Howard acted swiftly to ban semi-automatic weapons and Australians were with him."
Thirty-five people were murdered by Martin Bryant, who was firing two MSSA firearms, in Tasmania.
Just months after taking office, Howard moved quickly to push for new laws which banned automatic and semi-automatic weapons, established a comprehensive national firearm registration, and an amnesty period for prohibited and unregistered weapons to be surrendered.
In the "buy-back" scheme, the Australian Government purchased 700,000 firearms.
The attempts to reform gun laws in Australia were still controversial.
Hundreds of thousands of people protested the campaign; Howard even wore a bullet-proof vest when speaking to an angry crowd.
However, the new laws were changed and remain Howard's defining legacy.
A year later, a 1997 report by Sir Thomas Thorp also recommended tighter gun control in New Zealand.
One of the key recommendations was identical to Australia's response to Port Arthur: to ban MSSA weapons and make those firearms subject to a "buy-back" scheme.
The value of MSSA weapons in hunting and target practice was limited, Thorp said.
"I am satisfied that the potential consequences of MSSA misuse clearly outweigh any benefit to society in permitting their ownership."