New Zealand authorities say the country will be a safer place after gun owners handed in more than 50,000 firearms during a buyback program following a ban on assault weapons, but critics of the scheme say the amount collected has been woefully low.

The police then launched a six-month program to buy the newly banned weapons from owners.

The buyback ended midnight on Friday, with gun collection points staying open late as police reported a surge in last-minute returns.

Provisional figures indicate 33,000 people handed in 51,000 guns.

Another 5,000 guns were collected 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.

Provisional figures indicate 33,000 people handed in 51,000 guns.

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.

Key points:

• New Zealand's buyback scheme ended on Friday, after a six-month process
• Initial figures estimate that 33,000 people handed in about 51,000 guns
• Gun advocates say that up to two-thirds of the banned weapons have been retained
• Wellington banned the most lethal types of semi-automatic weapons less than a month after a gunman in March killed 51 people at two Christchurch mosques.

And police said they were in the process of collecting another 1,600 guns 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".

Both Mr Nash and Mr Clement said the country was now safer than it had been before the March attacks.

But critics said the process was flawed and some owners have reportedly illegally stashed their firearms.

Nicole McKee, a spokeswoman for advocacy group Council of Licensed Firearms Owners, said owners had kept about two-thirds of the banned weapons because they had lost faith in the Government and had not been offered adequate compensation.

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The council estimates 170,000 semi-automatic weapons were in circulation before the amnesty was announced.

"Despite our best efforts to encourage compliance, we know owners have been so disappointed by the settings of the ban and its poor implementation that many can't bring themselves to comply," Ms McKee said.

"[Gun owners] never overcame being blamed by authorities for being somehow responsible for a heinous act of terrorism — something they would never do."

Both Mr Nash and Mr Clement said the country was now safer than it had been before the March attacks.

The ban on assault weapons was strongly backed by lawmakers in a historic 119-1 vote after the mosque shootings.

MPs are now considering further restrictions, including creating a register to track all guns.

Police figures indicate the government paid out just over $NZ100 million ($95.7 million) to compensate owners during the buyback.

The Opposition National Party said the amnesty had collected less than a third of what police and firearm stakeholders had previously advised could be out there.

"The Government made a mistake in targeting law-abiding firearms owners when they should've targeted the gangs who peddle misery in communities across New Zealand," National Party spokesman Mark Mitchell said.