A gun law loophole which let the alleged Christchurch shooter easily upgrade his semi-automatic rifle into a military-style weapon was also exploited by the Northland siege gunman who killed two people.
Prime Minister Jacinda Ardern has promised New Zealand gun laws will change following the deaths of 50 worshippers at two mosques in Christchurch last Friday.
The 28-year-old Australian was able to legally purchase an AR-15 rifle with a standard A-Category licence, which he used to allegedly carry out the terror attack.
These firearms can only be sold with a magazine holding no more than seven rounds.
But anyone can purchase a high capacity magazine - which can hold 30, 75, up to 100 rounds - with or without a licence.
Slotting these unregulated parts into an AR-15 converts the firearm into a Military Style Semi Automatic weapon (MSSA), otherwise only be purchased with an E-category licence, which are much harder to obtain.
While upgrading a semi-automatic into a MSSA is a criminal offence, a police briefing to Police Minister Stuart Nash said the probability of discovering this was "negligible".
The alleged Christchurch shooter was seen with high capacity magazines and the same loophole was exploited by Quinn Patterson.
He shot and killed two women, Wendy Campbell and daughter Natanya, and wounded Jeff Pipe in July 2017.
Patterson built up an arsenal of weapons - which he called his "project" - and fired on the trio when they came to inspect the rural Northland property he was renting.
The unprovoked attack led to a siege involving the Special Tactics Group, the highly trained police unit, who came under fire from "heavy calibre" firearms.
Patterson set the house on fire and was later found dead among the charred rubble with 11 firearms and thousands of rounds of ammunition.
He did not have a firearms licence.
Instead, Patterson used the A-Category licence of his friend Michael John Hayes to order semi-automatic weapons through TradeMe.
The TradeMe purchases included four semi-automatic shotguns, four .22 calibre semi-automatic rifles, and two 7.62 calibre AK-47-type weapons.
Hayes, a former soldier, was sentenced to 12 months' home detention.
In a briefing to Police Minister Stuart Nash, police pointed out Patterson was able to exploit the "identified gap" in the law by converting the semi-automatics to MSSA with high capacity magazines.
Patterson's arsenal included a Saiga 7.62 fitted with a drum magazine capable of holding 100 rounds and an AK-47 replica with a flash suppressor, pistol grip - also restricted to MSSA - and a 30 round magazine.
Other high-profile crimes in recent years have featured A-Category semi-automatics upgraded to MSSA with larger magazines.
Rollie Heke fired on three police officers outside Morrinsville with a converted semi-automatic in 2017, while Rawden Yates used an AK-47 replica with a 75 round drum magazine to murder his friend Kimble Moore.
Politicians were told about Yates and the MSSA loophole at a Select Committee hearing investigating how guns were getting into the hands of criminals.
Noting concerns about "A category" firearms easily converted to MSSA, the 2017 report recommended police investigate a new semi-automatic firearm licence category.
This recommendation was one of 13 rejected by the then Police Minister Paula Bennett.
According to a briefing paper, her advisors Geoff Thomas and Nicole McKee said the proposal was outside the terms of reference for the inquiry and would not stop criminal possession of firearms.
"We are not happy with the 'risk' assessment. A competent firearm handler can dispatch rounds from any type of action just as quick as with a semi-automatic," they wrote in a paper obtained under the Official Information Act.
"We believe 'risk' is a 'perceived risk' of the tool when it is the user, not the firearm that needs the 'risk' assessment."
McKee is on the board of the Council of Licenced Firearms Owners [COLFO] which claims to be the largest pro-gun lobby group in New Zealand.
Around the same time, COLFO chairman Paul Clark was strongly lobbying Bennett about conflict between firearms owners and the police.
"As a member of the National Party Hutt Cabinet Club [a fundraiser], I find this conflict between police and firearms owners detrimental to the overall good of NZ society," Clark wrote.
"And being realistic [it] will not improve National's position in the forthcoming election. There are approximately about 242,000 licensed firearms owners in New Zealand, the vast majority of voting age.
Asked on Tuesday about the lobbying and her earlier decision, Bennett replied; "That's
the time that it was in. There is no point in having hindsight... we're in unprecedented times right now."
Prime Minister Jacinda Ardern is expected to announce gun law changes this week.
She has not ruled out a complete ban on semi-automatic weapons like Australia in 1996 after 35 people were killed in the Port Arthur massacre.
A compromise position would be to exclude shotguns and .22 calibre rifles, regularly used by hunters and farmers, from the ban.
John Howard, the then Australian PM, 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.
But the issue of illegal firearms has been a vexed issue in New Zealand since the Aramoana massacre in 1990, where David Gray used two MSSA firearms to kill 13 people.
However, a review by Sir Thomas Thorp made 60 recommendations in 1997 to tighten gun control in New Zealand.
One of those recommendations was to ban semi-automatic firearms.
But the Thorp Report was largely ignored and the National Government only partially introduced one of the recommendations in 1999.
"Tragically, I believe if the Thorp recommendations had been implemented in 1997, the Christchurch attacker would not have been able to obtain the semi-automatic weapons he used in this country," said Simon Mount, QC, who worked alongside Sir Thomas on the report.
"Without access to semi-automatics and ammunition, I believe the attack would not have occurred, or would at least have been made much more difficult and less deadly."