She says Cabinet has made "in principal" decisions about the reforms, and there will be change within 10 days.
She did not say what those changes would be, but said media would be told within a week.
"I know for a short period this might create a period of uncertainty for those who own guns for legitimate purposes. I particularly acknowledge those in our rural communities.
"I want to assure you that the work that we are doing is not directed at you.
"In fact I strongly believe the majority of gun owners in New Zealand will agree with the sentiment that change needs to occur."
A total of 50 victims have been confirmed dead after Friday's terror attack on two Christchurch mosques, with the dead ranging in age from 2 to older than 60
Many more were wounded, with 33 still in hospital.
Ardern says there will also be an inquiry, to find out what government agencies knew, or should have known, and whether the attack could have been prevented.
It will focus on the GCSB, SIS, police, and customs.
The Prime Minister also had a stern warning for social media companies, after an explicitly violent video of the shootings spread like wildfire online.
Jacinda Ardern says she's spoken to Facebook, and made it clear more can be done to control what is spread online.
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The signalled change in gun laws comes after it was today revealed Gun City sold four firearms to the alleged gunman.
Owner David Tipple said today he and his staff were "dismayed and disgusted" by Friday's shootings.
However, he said he did not feel responsible for the shootings.
He says the military-style semi-automatic reportedly used by the gunman wasn't bought from Gun City.
Tipple says all sales followed a police-verified online mail order process
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As conversations start around tightening our gun laws, there could be lessons learned from across the ditch,
In 1996, 35 people were gunned down at Port Arthur in Tasmania.
Within 12 months, Australia introduced a shooters licensing scheme, created a gun registration programme, and banned automatic and semi-automatic weapons.
Former National Party of Australia leader Tim Fischer was Deputy Prime Minister at the time.
He remembers strong opposition from some country areas.
Fischer says public meetings to explain why the changes were needed, turned the tide of public opinion.
Experts agree that changes to our gun laws are long overdue.
Otago University's Public Health Department is calling for an urgent and immediate government moratorium on semi-automatic imports and sales.
Researcher, Marie Russell, says firearms policy has been dictated for too long by an active and influential gun lobby.
She says there's no justification for having semi-automatics in our country.
TradeMe has already stopped selling semi-automatic weapons in the wake of the Christchurch mass shootings.
TradeMe chief executive Jon Macdonald said earlier today the company was waiting for more clarity from the Government, but has been contacted by lots of Kiwis over the weekend asking them to stop sales.
He said the company decided to listen to public sentiment.
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A South Otago rifle club has closed its range for the "foreseeable future" after learning the alleged mosque terror gunman was a member, and may never open again.
Brenton Tarrant was a member of the Bruce Rifle Club, and practised shooting an AR-15 and a hunting rifle at its range near Milburn.
Bruce Rifle Club vice-president Scott Williams says they closed their range in the wake of Friday's terror attack.
Asked if the range would ever reopen, he said: "We don't know at this stage."
Police visited the range yesterday with some of its members.
Williams would not comment on whether police were investigating claims by Pete Breidahl that he saw club members used the confederate flag - a symbol of white nationalism - and talking with "strong feelings" about the right to carry arms, as well as complaining that New Zealand's relaxed stance on refugees would lead to terror attacks.
"At the suggestion of police they have asked us not to comment on those allegations."
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The attack is being described as one on all New Zealanders, not just the Muslim community.
That's from the Imam of the Al Noor Mosque, where 42 worshippers lost their lives during Friday prayer.
Gamal Fouda says the attack was a war on peace.
Fouda says he's not going to let the attack scare him, or stop him loving New Zealand.
Two of those who died were students at Cashmere High School. Year 10 student, Sayyad Milne, and Year 12 student Hamza Mustafa were killed.
Cashmere High School principal Mark Wilson says the school began the day with three separate assemblies for its 2000 plus students, to acknowledge the tragedies and give students and staff a moment to come together collectively.
The school's head boy Oki Tilaia invited students from all over Christchurch to unite at the Deans Avenue memorial they held at 4pm.
Schools throughout Christchurch were on lockdown through Friday, following the mass shooting.
Today Traumatic Incident teams are visiting schools to offer advice and support
Waimairi School Principal Mike Anderson says number one today is assuring children they are safe.
The Ministry of Education says support is available for schools, kura and early learning services.
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St John Ambulance staff who responded to the terrorist attack on Friday have spoken of the horrific conditions they saw, and the pride they feel from being part of an efficient response that is believed to have saved lives.
A visibly distressed Paul Bennett gave a chilling description of the scene at the Deans Ave mosque.
He also worked through the Christchurch earthquakes, but described Friday as the most horrific thing he'd ever seen.
Bennett says the shooting was about hatred.
"There was a lot of blood, a river of blood coming out of the mosque.
"That's a scene you don't forget. It was literally flowing off terracotta tiles, amongst fatalities."
He became emotional and fought back tears as he recalled the "most horrific" scenes he had ever witnessed.
St John Territory Manager Craig Downing says they've learnt from international tragedies it's crucial frontline workers get support services as soon as possible.
Downing says peer support and a clinical psychologist are on hand for staff.
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Accused mosque gunman Brenton Tarrant has dismissed his lawyer and plans to represent himself in court - raising concerns he could attempt to turn a trial into a platform for his beliefs.
The duty lawyer who represented Tarrant in court on Saturday confirmed to the Herald today that he was no longer acting for him.
Richard Peters says Tarrant appears to be lucid and not mentally unstable – other than the extreme views he holds.
Peters said that his job representing Tarrant ceased on Saturday - and that the accused gunman had told him he wanted to represent himself in future.
This raises the prospect of Tarrant conducting his own defence at trial and using the high-profile prosecution to promote his beliefs, which were detailed in a manifesto before Friday's shootings.
Tarrant is facing one charge of murder, but is likely to face more. He was remanded in custody to appear in the High Court on April 5, and did not apply for bail or name suppression.
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Tarrant has been warned he's a marked man in prison.
Gang members gathered at Hagley High School in Christchurch yesterday to take flowers and offer their support to families affected by the tragedy.
They said what Tarrant allegedly carried out was "wrong in every way possible".
Speaking to the Herald, one man said the gang members had gone to the school to tautoko (support) the families.
Another man told the Herald, "we've got friends inside too." He didn't elaborate but the warning was clear.
Criminal justice advocate Sir Kim Workman says he's heard from an independent source that Tarrant could be in danger in prison.
He says feelings can run high in jail, and Tarrant would likely be kept in solitary confinement for his own safety.
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Tarrant's family has spoken out to say that they're shocked, and what he's done is unforgiveable.
His 81-year-old grandmother Marie Fitzgerald, and uncle Terry Fitzgerald, have told Nine News they learned it was one of their family while watching the news.
At first they didn't believe it.
"We're all gobsmacked, we don't know what to think. The media saying he's planned it for a long time so he's obviously not of sound mind I think," she said.
"We're so sorry, for the families over there, for the dead and the injured ... [we] just want to go home and hide.
"It's just so much of everything to take in that somebody in our family would do anything like this."
Following Tarrant's arrest, his mother and sister are reportedly staying in a safe house in Australia and assisting authorities.
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Australian counter-terrorism police have raided two homes in New South Wales connected to Brenton Tarrant, including his sister's home.
The Australian Federal Police and NSW Police released a joint statement about the raids.
"The primary aim of the activity is to formally obtain material that may assist New Zealand Police in their ongoing investigation."
Officers said there was no impending threat related to the search warrants, and Tarrant's family is helping police with their inquiries.
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A major question following this violence is what part social media played in the tragedy.
Facebook said that it removed 1.5 million videos of footage from the shooting rampage at two mosques in Christchurch within 24 hours of the attack.
However, that only underscores the massive game of whack-a-mole social media giants have to play with even the most high-profile problematic content on their platforms.
In a statement, Facebook New Zealand spokeswoman Mia Garlick said the company was continuing to "work around the clock to remove the content" using a combination of technology and people.
Of the 1.5 million videos of the massacre, filmed by a body-worn camera on the perpetrator almost in the style of a video game, 1.2 million were blocked at upload.
However, that leaves 300,000 videos that had to be chased down, and removed once people had already seen them.
Over the weekend Prime Minister Jacinda Ardern said there were "further questions to be answered" by Facebook and other social media sites over their response to the events.
Brenton Tarrant's Facebook followers were the first to know about the shooting. He live-streamed his assault, from the time he started driving to Al Noor Mosque to the moments when he fired his first shots.
Many hours later, and long after he and other suspects had been arrested, others were still uploading the video to YouTube and other online video platforms.
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An 18-year-old accused of sharing a live-stream of the mass shooting has been refused bail.
The teen faces two charges: one of sharing the gunman's live-stream and a second for posting a photograph of one of the mosque's attacked with the message "target acquired" along with other chat messages "inciting extreme violence".
The 18-year-old appeared in Christchurch District Court today and was granted name suppression. However his request for bail was refused by the judge.
Police have said the teen - who cannot yet be named - was not involved in the shootings on Friday.
The maximum sentence for each of the charges against the 18-year-old is 14 years' jail.
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Some of New Zealand's biggest brands are set to pull ads from Facebook and Google after the massacre.
The Herald understands that ASB, Kiwibank Lotto NZ, Burger King, Spark and number of other companies have banded together to take a stand against the harm caused by unmoderated content.
It's understood that marketing managers from the different companies spoke over the weekend about what they could do promote change in the industry.
At this stage, it is still unclear how extensive the pull-back will be or for how long the companies are likely to pull their digital ads.
Lessening the flow of money into these companies is the one way in which companies can get an immediate reaction from the big tech companies – as illustrated by YouTube's decision to ban all comments on content featuring minors after brands took exception to the unsavoury remarks often made.
A joint statement today from industry bodies the Association of New Zealand Advertisers (ANZA) and the Commercial Communications Council (Comms Council) called on social media platforms to do more to ensure that such horrors are never again live-streamed.
ANZA chief executive Lindsay Mouat today told the Herald that New Zealand companies seriously need to consider whether they want they advertising money spent.
He believes New Zealand's businesses have a role to play in encouraging change.
"Perhaps when the dollars start to go, you'll get a response."
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Others think we need to better enforce our laws - to control online hate speech.
Germany passed a law in 2017 requiring platforms to erase hateful speech posts within 24 hours, under pain of fines of up to 50 million euros.
Security company SafeStack director, Laura Bell, says those laws are enforced and the crimes are taken extremely seriously.
She says New Zealand's laws are weaker, and it would require a cultural shift to hold online platforms to account, not just individuals.
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Kiwis across the country are flocking to makeshift Christchurch shooting memorials as they leave flowers, donate money or simply offer hugs to members of New Zealand's Muslim community.
Auckland-based Dr Anwar Sahib, the chairman of NZ Islamic Information Centre, was among the "traffic jam" of people gathered near Christchurch Al Noor Mosque today to pay their respects to victims of Friday's terror attack.
He was joined there by ordinary Kiwis, Islamic leaders and even a high-level Turkish delegation.
Sahib says he and other members of the Muslim community had been flooded with support since a gunman killed 50 people and injured countless others.
His NZ Islamic Information Centre has now collected almost $2 million for the families of victims through a LaunchGood fundraising page.
A separate Givealittle page set up by the Victim Support group has raised $5.2m.
Yet one of the most touching aspects of the response to the attacks had come from ordinary Kiwis on the street.
"On Saturday, when I was at the petrol station filling up my car, a young man walks to me, shakes my hand, hugs me and walks away - we didn't even need to say words to each other," Sahib said.
"This is the kind of support we are getting."
Meanwhile there was a huge turnout at Wellington's Basin Reserve last night to stand shoulder to shoulder with the Muslim community.
The overwhelming sentiment from the thousands who went was that nothing will divide New Zealanders.
There have been vigils at cities and towns around the country since Friday's attack.
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That's the Front Page for today, Monday, March 18, 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.
You can subscribe to this podcast on Apple podcasts here, iHeartRadio here, and Stitcher here.
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