The government banned the most lethal types of semi-automatic weapons less than a month after a gunman killed 51 worshippers at two Christchurch mosques in March.
Police then launched a six-month programme to buy the newly banned weapons from owners.
The buyback ended at midnight on Friday, with gun collection events staying open late as police reported in a last-minute surge.
Provisional figures indicate 33,000 people handed in 51,000 guns, and another 5,000 weapons as part of a parallel amnesty in which owners could hand over any type of firearm without any questions being asked but without getting compensated.
Owners also modified another 2,700 guns to make them legally compliant, while police said they had seized a further 1,800 guns from gangs since March. Officers said they are also in the process of collecting another 1,600 weapons from gun dealers.
Police minister Stuart Nash told reporters on Saturday that criminals would find it harder to get their hands on assault weapons because they tended to steal them from lawful owners, but those weapons would now be out of circulation.
Police deputy commissioner Mike Clement thanked gun owners for doing the right thing. He acknowledged in a statement it had been “a difficult process for some people”.
Mr Nash and Mr Clement said the country is now safer than it was before the March attacks, but critics say the process was flawed and many owners have illegally stashed their firearms.
Nicole McKee, a spokeswoman for the advocacy group Council of Licenced Firearms Owners, said owners had kept about two-thirds of banned weapons because they had lost faith in the government and had not been offered adequate compensation.
“They never overcame being blamed by authorities for being somehow responsible for a heinous act of terrorism - something they would never do,” Ms McKee said in a statement.
The ban on assault weapons was strongly backed by legislators in a historic 119-1 vote after the mosque attacks. Legislators are considering further restrictions, including creating a register to track all guns.
Police figures indicate the government paid just over 100m New Zealand dollars (£50m) to compensate owners during the buyback.
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