A new option allows owners of some prohibited firearms to have them modified by approved gunsmiths to make them lawful, with costs up to NZ$300. The police expect that tens of thousands of guns will be surrendered, although the exact number is unknown.
Regional police commander Mike Johnson said 903 gun owners in the Canterbury region, which includes Christchurch, had registered 1,415 firearms to be handed in.
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.
The government has put aside more than 200 million New Zealand dollars (133.90 million US dollars) for payments and administration of the gun buyback and amnesty.More news: Pres. Trump tells liberal Democratic congresswomen they should leave the US
Under an amnesty, gun owners have until December to turn over their now-banned weapons.
Police Minister Stuart Nash said the objective was to "remove the most unsafe weapons from circulation".
"They do want to abide by the new laws but they have no incentive and they're having fingers pointed at them and are being treated like criminals", McKee said. The amount each individual will receive will depend on the value and condition of their guns.
The Council of Licenced Firearms Owners this week said it would join a fight against the legalisation and the buyback scheme.More news: National Football League star Josh Norman takes part in Pamplona bull runs
The royal commission - the most powerful judicial probe available under New Zealand law - will examine events leading up to the March 15 attack in which a lone gunman opened fire on two mosques in a mass shooting that shocked the world.
"We needed these semi-automatic assault rifles out of the community, but it's appropriate that people who have had to hand them in are compensated for it", he said.
There are concerns too that farming communities, which rely on firearms for hunting and pest control, will suffer because of the weapons ban. It prompted more than 700,000 weapons to be surrendered.
He has pleaded not guilty to terrorism charges, as well as 51 counts of murder and 40 of attempted murder. He is expected to go on trial next year.More news: Pakistani terrorists move to Afghanistan after Balakot strike
Mr Nash urged more gun owners to head to Riccarton Racecourse tomorrow for the second day of the buy-back event.