She says people are asking how the attacks, in which 50 people died and dozens more injured, were able to take place, including how the alleged gunman obtained the weapons, the role of social media and the role of agencies.
Ardern says she also has questions.
The inquiry will focus on whether security agencies were focused the right way and whether there were any clues that were missed.
Ardern says the inquiry will look at events leading up to the attack rather than the response.
The terms of reference have not yet been drawn up, or the names of the inquiry members released.
The Prime Minister says she and other Ministers are also meeting this afternoon with Brad Smith of Microsoft to discuss the role of social media following the sharing of livestreamed video of the alleged gunman's actions.
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Today the Privacy Commissioner John Edwards has delivered Facebook another serve, saying the social media giant's silence is an 'insult to our grief'.
Edwards sent an email to Facebook executives on Friday, that he has shared with the Herald today.
In it, he says: "It would be very difficult for you and your colleagues to overestimate the growing frustration and anger here at Facebook's facilitation of and inability to mitigate the deep, deep pain and harm from the live-streamed massacre of our colleagues, family members and countrymen broadcast over your network.
"Your silence is an insult to our grief."
Edwards has also expressed frustration that Facebook executives previously promised open communication channels, but that they've ghosted since the terror attacks, otherwise known as ignoring all communication.
Problems include a New Zealand hate group remaining on Facebook, complete with images too offensive to publish, despite Herald reporting on the matter.
The Privacy Commissioner says he had hoped for communication with Facebook after the Christchurch shootings, especially given he has publicly asked the network hand over account details to police of all people who shared the gunman's video.
He says sharing the video is an "egregious" violation of the victim's privacy, as well as our censorship laws.
The social network has not responded to Edwards' request, but in an interview with the Herald, Facebook's VP for global policy Monika Bickert indicated it would not cooperate.
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An Al Noor Mosque shooting survivor is looking for the woman who saved her and her baby's life, hoping she can thank her.
Majda El Hajji and her family were among the hundreds of people who turned out to Christchurch's biggest mosque for Friday prayers on March 15.
She had brought along her 5-month-old son for one of his first visits to the mosque and said he'd drawn quite a crowd of people, fussing and cooing over him.
But El Hajji says everything changed the instant they heard shots, and ran for their lives.
A woman was shot in front of her, and the gunman was firing at her as she fled.
Then out of nowhere – help appeared.
El Hajji says a woman pulled her car up next to her and let them in – while she was still fleeing the gunfire.
She says as soon as the woman heard what was happening, she pulled over and let two more Muslim women in.
Now, El Hajji says she wants to be reunited with the woman, in order to thank her properly.
"I want to say thank you to all New Zealanders. Thank you for helping us. Thank you for supporting us. Thank you for showing us we are not different – we are all the same."
El Hajji says she doesn't remember what kind of car the woman had, but she was wearing some kind of food-related work uniform.
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The vice-president of the Otago gun club where the alleged terror attacker trained has contacted police to hand in his now-illegal weapon.
Prime Minister Jacinda Ardern announced a ban on military-style semi-automatic (MSSA) firearms and assault rifles on Thursday afternoon.
Bruce Rifle Club vice-president Scott Williams said he filled out an online form that evening, notifying police he wished to hand in a firearm.
Williams said the range was closed in the wake of the attack, and he is unsure whether it will ever reopen.
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It seems many New Zealanders unwittingly have their retirement funds invested in weapons.
Of the 254 Kiwisaver funds, only 16 are weapons-free.
Research by KiwiSaver provider Simplicity has found five of 21 KiwiSaver providers have specific bans on investing in firearms manufacturing companies.
Those providers are Nikko, Juno, Amanah, KiwiWealth and Simplicity.
Outside of the five, Booster, Quay Street and SuperLife offer specific ethical or responsible investment funds which screen out all armaments investments.
The Herald took the opportunity to ask 16 major KiwiSaver providers whether they had any direct or indirect investments in guns or munitions, and what they planned to do about it.
If you're interested in seeing their full responses, click here
Mainzeal's Richard Yan is appealing the High Court ruling which demanded he and other directors pay $36 million.
It's not yet known whether former Prime Minister Jenny Shipley, former Mainzeal chief executive Peter Gomm and director Clive Tilby will join the appeal.
Mainzeal went into liquidation in 2013, with losses of $110 million.
No dates have been set for it to go to the Court of Appeal, but it's not expected before early next year.
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Teen killer Haami Hanara's father has admitted hiding his son from police.
Witerangi Hanara told police and social workers he didn't know where his 14-year-old son was after the teen stabbed Flaxmere man Kelly Donner last year.
But he was hiding him all along.
Hanara, 43, pleaded guilty in the Napier District Court this morning to perverting the course of Justice.
He'll be sentenced in May.
His son Haami Hanara was found guilty of fatally stabbing Kelly Donner last year.
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New data showing that more than 43,000 Aucklanders are directly threatened by rising seas has prompted warnings that in future home insurers may be forced to hike prices - or withdraw coverage altogether.
An Auckland Council report has found between 1.5 per cent and 4.5 per cent of the city's land area now sat in low-lying coastal areas that could be vulnerable.
That equates to about 43,000 people.
It's not just peoples' homes in the firing line: up to 6 per cent of council-owned greenspaces are threatened - including 55 cemeteries – as were 3 per cent of buildings and dozens of public facilities.
About 1 per cent of roads in the region are threatened by a sea level one metre higher – and some areas of Auckland International Airport are affected under every scenario.
The picture is also dire for rural coastal areas – and there is a risk that 5 per cent of the region's most fertile land could be at risk as early as the middle of the century.
Insurance Council chief executive Tim Grafton says: "if we continue to do nothing, then insurers will respond by deciding whether they want to accept those risks."
That could mean higher prices, higher excesses, or refusal to offer insurance at all.
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A rare whale species that was nearly hunted to extinction has been found to hang around Southern Ocean shores more often and longer than first thought.
The discovery was made using technology that could be crucial to helping the southern right whale's recovery, after a year-long study using acoustic monitors.
Once abundant around New Zealand coasts, the ocean giants are now mainly found at their remote breeding ground, the Auckland Islands, some 460km south of Bluff.
It was estimated there were 30,000 before whaling in throughout the 1800s devastated their population, which has returned from the brink to around 2000 individuals today.
This new scientific study found they found they could hear their calls in every month except January, and the calls peaked during the winter months when the whales move near the shore to calve.
The most common calls are upcalls, a deep, rising "whoop" that lasts about a second, which are thought be a type of "contact call" - a way of letting other whales know who is around.
Otago University marine conservation biologist Professor Steve Dawson says there was little reliable information on how their vocalisations varied throughout the year, how they used different types of vocalisations, and the rate at which they called.
He says the study is important for understanding southern right whales, but also pioneering scientific efforts to monitor areas that are difficult to get into, and over long periods of time.
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That's the Front Page for today, Monday, March 25, making sure you're across the biggest news of the day. For more on these stories, check out The New Zealand Herald, or tune in to Newstalk ZB.
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