Prime Minister Jacinda Ardern has spoken of the moment she decided that gun laws had to change, and her shock at how easily it was to get destructive weapons in New Zealand.

In a sign of the importance of gun law reform, Ardern took the unusual step this evening of speaking at the third reading of the Arms (Prohibited Firearms, Magazines and Parts) Amendment bill, which is in the name of Police Minister Stuart Nash.

After an expedited process, the bill passed tonight with the support of all parties except for Act.

Ardern told the House about a briefing she had with Police Commissioner Mike Bush shortly after the terror attack on March 15, when he told her the gunman had obtained his firepower legally.

"I could not fathom how weapons that could cause such destruction and large-scale death could have been obtained legally in this country. I could not fathom that," Ardern said.

"I could not hand-on-heart go down and face not just the media, not just the public, but the victims that had been left behind from this terror attack and tell them hand-on-heart that our system and our laws allow these guns to be available and that was okay. Because it was not.

"I made a decision after that briefing that I would go down that day and, without having the chance to question the Parliament, know that Parliament would be with me. And they were."

Ardern paid particular tribute to the National Party for supporting the bill.

She also defended the haste of the bill's passage, and spoke of visiting the victims in hospital.

"I struggled to recall any single gunshot wounds. In every case, they spoke of multiple, debilitating injuries ... We are here because of them. I believe they are here with us.

"Fifty people died and they do not have a voice. We in this House are their voice. And today we have used that voice wisely."

The bill is expected to receive its Royal Assent tomorrow, and on Friday military-style semi-automatics (MSSAs) and assault rifles and associated parts will be illegal.

Possession of prohibited firearms could see a jail term up to five years.

There are some narrow exemptions including for pest control, collectors, heirlooms and mementos.

The National Party tried to add Firearms Prohibition Orders and an exemption for competitive shooting to the bill, but these were blocked by Labour, NZ First, and the Greens.

Nash has said those issues may be considered later this year in a second round of gun law reform.

Owners of prohibited firearms and parts will have until the end of September to hand these in to police, though the Government can extend the amnesty period.

The bill also includes a regulatory framework for the buyback scheme.

Compensation will be available for firearms and parts based on make, model and condition, but only for items that were lawfully obtained by people with the appropriate firearms licence, "or persons lawfully in possession of prohibited items".

That means that gun dealerships could be compensated for firearms and gun parts, but they may miss out for any business losses.

A process for appeal will also be available.

The details for the buyback scheme are still being worked through and will be considered by Cabinet in May.

Act leader David Seymour warned that restricting compensation could discourage people from handing in their firearms.

"If a significant number of weapons aren't handed in, we risk creating a large black market of dangerous weapons without any regulatory oversight. That may be a more dangerous world than we had prior to March 15."

The passage of the bill brings to an end the first phase of gun law reform, which has seen emotional pleas in support of the law from the Muslim community, broad support from hunters and farmers who sought wider exemptions to the ban, and vociferous opposition from gun lobby groups that claimed the ban was too harsh on law-abiding gun owners.