One expert believes a generous buy-back scheme will be a strong incentive for the voluntary surrender of banned firearms but warns of the 'true loner'.
Tracking down the newly outlawed semi automatic firearms may be a case of history repeating itself.
Following the Aramoana massacre in 1990, in which loner David Gray gunned down 13 people, the National Government of the day moved to ban "Rambo" style guns.
A new category of firearms called Military Style Semi Automatic (MSSA) was created and anyone who wanted to own them needed an E-category licence, which was harder to obtain.
Any MSSA - with the defining features a "free-standing pistol grip" and magazines holding more than 7 rounds - also had to be registered.
Today, there are 13,500 registered MSSAs in New Zealand held by 7500 E-category licence holders.
But not everyone put their MSSA on the list.
When Jan Molenaar shot dead Constable Len Snee - then himself - in the Napier siege in May 2009, police found an arsenal of 14 high-powered weapons.
One of them was an L1A1 Self-Loading Rifle.
The Defence Force sold 3000 of these firearms in the late 1980s before Aramoana and subsequent MSSA classification.
"Very few [L1A1] are registered with police," according to a briefing to Police Minister Stuart Nash released under the Official Information Act.
"One L1A1 was found in Jan Molenaar's arsenal and another seized in 2016 from a methamphetamine operation in Christchurch."
Where the rest are, nobody knows.
It was a point picked up by the Coroner investigating the deaths of Molenaar and Snee.
The very first recommendation of Coroner David Crerar was New Zealand's gun laws needed to change.
"The policy of tracking MSSAs and confirming the type of firearm that is a MSSA must be looked at again," Coroner Crerar wrote in August 2010.
"The Thorp Report may have to be revisited. At present the Arms Act is only complied with by honest people."
His findings were ignored. As was the Thorp Report in 1997 and the recommendations of a Law and Order select committee inquiry in 2017.
Nearly 10 years after the Napier siege, gun laws are changing.
Following the Christchurch terrorist attack, the Government moved quickly to reclassify semi-automatic weapons with a calibre greater than .22 as MSSA.
These weapons, such as the AR-15 used to shoot dead 50 innocent, were available on standard A-category licences but now need the stricter E-category endorsement.
Within weeks, all MSSAs will be illegal.
Parts such as high-capacity magazines, which were available to anyone with or without a licence, will also be banned.
The pending ban follows, finally, one of the 60 recommendations made by the Thorp Report in 1997.
All of them were ignored.
The Government should consider another of the shelved recommendations: a national register of firearms.
It wouldn't magically fix the problem overnight, but build a more detailed picture over time.
At the moment, no one has any idea how many semi-automatic firearms are now classified MSSA.
That's why the cost of the buy-back scheme - where owners are paid for surrendering weapons in an amnesty period - is estimated from anywhere between $100 and $200 million.
There's also no idea figure of how many semi-automatic weapons are in the hands of criminals.
Incredibly, while it's a criminal 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.
Not even the licence number they've supposedly been shown. It could be fake or non-existent.
"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," the briefing to Nash said.
"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 other words, the lack of existing records - as Coroner Crerar pointed out - means police will still rely on the honesty of gun owners.
The number of guns surrendered so far has not been tallied up nationwide, according to a spokeswoman for Police National Headquarters.
Asked how police planned to track down unlawful, unregistered firearms once the amnesty ends, the spokeswoman said: "We are currently working through how we will manage that process."
Honesty shouldn't be underestimated, says gun policy researcher Philip Alpers - especially when coupled with generosity.
When Australia banned all semi-automatic firearms in 1996 following the Port Arthur massacre, Alpers said there four states or territories that did not have a firearms register like New Zealand..
"The generous buy-back scheme pulled most of the firearms owners out. They got a good price, many were overpaid for a ratty old firearm, much more than on the open market," said Alpers.
"So there was a considerable incentive to get rid of them."
Alpers also believed the small-town nature of New Zealand, as well as the tendency of hunters and shooters to talk openly about their firearms, means those with large collections will likely be dobbed in to police.
"Nothing goes unnoticed in New Zealand. With any luck, we'll only be left with the guy who had four MSSA and decides to keep one," says Alpers.
"And the true loners. The Jan Molenaars. They're the ones we need to worry about."