"So in terms of the sort of gun the terrorist used to kill people on March 15, we have no idea whatsoever", Mr Nash said.
Licenced firearms owners have time until December 20 to surrender weapons that have now been deemed illegal under the scheme.
Gun owners are planning to stay home en masse today as frustration builds around the first buyback event in Christchurch. The police expect that tens of thousands of guns will be surrendered, although the exact number is unknown.
Some owners have decried the pricing list that determines how much will be paid out for weapons, and the process behind it, but police have said they will not be budging.
Under the buyback scheme, gun owners are compensated between 25% and 95% of the pre-tax price of a new gun, depending on the condition of their weapon.
The ban, which Prime Minister Ardern announced Thursday in Wellington, includes high-capacity magazines, which can hold multiple rounds of ammunition, and accessories that can convert ordinary rifles into fast-acting assault rifles.
But he said he couldn't fault how police were handling the new and unique situation.
Others at the firearms collection at Riccarton Racecourse said the process was simple, but were steadfast in disappointment. The law changed and now they hold prohibited weapons.
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But some owners are complaining that the compensation is inadequate.
Australian-born Brenton Tarrant has been charged with the killings and is alleged to have used an arsenal of five weapons, including two military-style semi-automatic rifles, in the attacks on the two mosques.
A judicial inquiry into whether New Zealand's police and intelligence services could have prevented the Christchurch mosque attacks in which 51 worshippers died began taking evidence on Monday.
Including guns given up during a test on Friday, police have now paid 378 weapon owners in Canterbury a total $1,022,599 for weapons prohibited by a law change introduced after the Christchurch mosque shootings.
There are concerns too that farming communities, which rely on firearms for hunting and pest control, will suffer because of the weapons ban. There are an estimated 1 million to 1.5 million guns in New Zealand and 250,000 licensed gun owners.
"But it really is very hard to even put a forecast in place until we have a better understanding of how many weapons are out there". He has pleaded not guilty to 51 charges of murder, 40 charges of attempted murder and one charge under the Terrorism Suppression Act. He is expected to go on trial next year.
A total of 224 guns were handed in today at the Government's first buyback event.