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People were asking how the attack was able to take place, including how the alleged gunman obtained the weapons, the role of social media and the role of agencies, she said.
Ardern said she also had questions.
The inquiry would take in the agency review already announced.
There would be a focus on whether security agencies were focused the right way and whether there were any clues that were missed.
Royal commissions were reserved for the gravest of events and the shootings was one of those, she said.
Ardern said the inquiry would look at events leading up to the attack rather than the response - that would come later.
The Prime Minister said the seriousness of the attack and the need for answers needed to be weighed up. That would be done when the terms of reference were drawn up, and the names of the inquirers released.
Ardern said she and other ministers would meet this afternoon with Brad Smith of Microsoft to further discuss the role of social media following the sharing of livestreamed video of the alleged gunman's actions.
Foreign Minister Winston Peters, who appeared at the post-Cabinet press conference alongside Ardern, said his focus during his recent visit to Turkey was to ensure the safety of New Zealanders travelling there and he was pleased by the response of President Recep Tayyip Erdogan.
Meanwhile, Ardern will be travelling to China on a diplomatic mission this Sunday but has scaled her trip back because of the terror attacks in Christchurch.
She will only be on the ground for a day and will be back in New Zealand on Tuesday.
This comes after plans for Ardern to visit Beijing were put on ice last year, sparking some concerns of diplomatic tension between the two nations.
Speaking at her post-Cabinet press conference today, Ardern said she would be in Beijing for meetings with the Chinese President Xi Jinping and Premier Li Keqiang on Monday.
She will formally open the New Zealand embassy.
Ardern said it was a visit that was planned "some weeks ago".
"But given the terrorist attacks in Christchurch, I cut it back to one day of meetings."
She said it was intended to be a longer visit and to include a business delegation – "but under the circumstances that just didn't seem appropriate".