It was estimated the gun buyback scheme could cost up to $149 million, but with accessories included, it doubled to up to $249 million.
Details of how the Government's gun buyback scheme was estimated, the decisions made by ministers, as well as discussion around the amnesty period, have been released by the police.
In one briefing on the financial implications of the scheme, it was estimated the cost could be between $48 million and $149 million, but with accessories included, it doubled to between $79 million and $249 million.
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Documents show how the cost of the buyback was settled on, by estimating how many guns were in circulation, estimating the market value of those guns, and how many could be expected to be handed in.
For example, it was estimated about 2 percent of the approximately 379,405 shotguns would be banned under the Government's new gun law reforms.
It was estimated that, if 5 percent of those guns were in low condition, 70 percent were in medium condition, and 25 percent were in good condition, it added up to $8.35 million.
The estimation was different for military-style semi-automatics which are all banned. Based on 50 percent being in good condition, 30 percent in medium condition, and 20 percent in good condition, the estimated cost was $12.14 million.
But that didn't include accessories. Another document highlights how that $12.4 million figure jumped to $20.71 million when accessories were included.
The same thing could be seen with rifles. It was estimated that if 20 percent of rifles were banned, it would cost $129 million. But with accessories added, it came to about $220 million.
The documents also reveal Treasury was asked for advice on the gun buyback pricing. It told ministers the "better approach" would be to "base all price categories on second-hand values, depending on the age and condition".
The Government set aside $208 million for payments and administration of the gun buyback scheme. It noted that if necessary, it will top up that figure.
It ended up agreeing to pay 95 percent of the 'base price' for banned weapons in new or near-new condition; 70 percent for used, and 25 percent if they were in poor condition.
These options were recommended by the police and consultancy firm KPMG.
Buyback cost 'up to $752 million'
In KPMG's firearms buyback pricing report, it was estimated that the cost of the buyback could be as high as $752 million. That was based on firearms handed in each getting $4331.
The report noted, "One of the challenges in determining the overall cost of the buyback is a lack of data relating to the actual firearms that are in public ownership."
The $4331 is much higher than the average price gun owners have been paid.
As of August 1, 6831 firearms had been handed in from 4200 people, with over $12 million paid in compensation. There are 16 collection events to be held over the weekend.
Police suggested that a 12-month amnesty period for the buyback - the same length as Australia's in 1996 - would not be "feasible or desirable" as they would expect a "large surge" of hand-ins at the end of the period.
The Government opted for an amnesty period of nine months, starting March 21 and ending December 20.
That was part of the first tranche of legislation passed under the Arms Amendment Bill. Last month the Government proposed more gun law changes, including establishing a firearms register.
The details can be read here.
The ban on all military-style semi-automatics and assault rifles was announced by Prime Minister Jacinda Ardern on March 21, days after the March 15 Christchurch terror attack.
The total number of all weapons in New Zealand, both lawful and prohibited, is estimated to be approximately 1.2-1.5 million.