Jacinda Ardern says she doesn't understand the United States' inability to change gun laws.
The Prime Minister was speaking to CNN's Christiane Amanpour in Paris as she leads the Christchurch Call summit, which will go over how social media can limit expressions of violent extremism.
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Fifty-one people were killed after a gunman opened fire inside the Al Noor (Deans Ave) and Linwood Ave mosques on March 15.
Military-style semi-automatics were banned in New Zealand on April 12, less than a month later.
Amanpour asked Ardern about New Zealand's 26 day turnaround on gun control legislation, which Ardern said was a natural response to the country's depth of feeling after the attack.
But she can't work out why some others haven't done the same.
"Australia experienced a massacre and changed their laws. New Zealand had its experience and changed its laws. To be honest, I do not understand the United States."
Ardern said New Zealand's gun laws were too permissive before the attack, and while there is still a use for some guns in New Zealand, there is not one for military style semi-automatics.
"I remember very distinctly sitting in a meeting where the police told me the weapons that had been used on the 15th of March and how easily they were legally obtained and then illegally modified for the purposes of that attack.
"There was no question in my mind that our laws needed to change in that moment. What I'm heartened by is that I did not question for a moment that Parliament would support that law change."
Gun law changes is one of several of Ardern's responses to the attack, she has also put social media companies on notice after the alleged gunman used it to share his violent white nationalist views.
He also livestreamed the shooting on Facebook.
Ardern revealed to Amanpour she has spoken to Mark Zuckerberg twice in the lead up to the summit, and he has given Facebook's support for the call to action.
But he won't be attending and has sent Facebook's head of global affairs instead.