Mr. Alpers said the challenge for New Zealand would mainly be getting the ammunition and guns that already exist out of circulation. Half of Australia’s states had some kind of gun registration plan in place before the 1996 reforms, making it easier for the authorities to know what weapons were out there and what needed to be brought in.

New Zealand only registers 4 percent of its weapons. According to the police, about 250,000 people in the country own an estimated 1.2 million to 1.5 million firearms. It is unclear how many of them would be affected by the ban.

“New Zealand is at a considerable disadvantage to countries that have had registries, because there’s no way of tracing the firearms because they don’t know who’s got them,” Mr. Alpers said. “We’re relying entirely on the honesty of the gun owner to turn it in.”

Ms. Ardern said that fair compensation would be paid to all those who participate.

Noting that there would be some limited exceptions for specific purposes, especially in rural areas, she said she expected the new law to be in place by April 11, the end of Parliament’s next session.

In the interim, as of Thursday afternoon, a change in regulations would alter the licensing rules for the weapons that would eventually be banned. To avoid a rush on purchases, weapons that now require a basic A Class license will fall under an E Class gun license, which is already much harder to obtain, and which the prime minister said would now be impossible to get.

“I can assure people there is no point in applying for such a permit,” she said.

[Read more about the victims of the attack, who spanned generations and nationalities, and their families’ struggle for closure.]