Ahead of a summit on online extremism, Ardern was responding to a question by CNN's Christiane Amanpour asking whether countries can learn from New Zealand.
The Prime Minister said guns have a "practical purpose" in New Zealand but "that does not mean you need access to military-style semiautomatic weapons and assault rifles."
"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," she said.
She then spearheaded a call for the world's biggest internet platforms to remove extremist content online after a livestream of the suspected gunman's attack was shared on Facebook, YouTube and Twitter.
The summit in Paris begins on May 15 and was organized by Ardern and French President Emmanuel Macron. It will see world leaders and tech executives sign a pledge called the "Christchurch Call," which aims to end the use of social media for acts of terrorism.
Ardern told CNN on Tuesday that the meeting "is not about regulation, it is about bringing companies to the table," adding that Facebook CEO Mark Zuckerberg has given "Facebook's support to this call to action."
The focus will "very much be on violent extremism," she said. The pledge will not limit or curtail "the freedom of expression."
"When it came to the way this attack was specifically designed to be broadcast and to go viral, (responding) to that needed a global solution, so that was why we immediately got in contact with international counterparts," Ardern told CNN.