Ardern described the event as extreme and unprecedented violence.
Police Commissioner Mike Bush has confirmed multiple fatalities.
He said there was one person in custody but they were unsure if there were other people.
Click here to see the Prime Minister's media conference
The horrific shooting was livestreamed for 17 minutes by the gunman.
The shooter identified himself as "Brenton Tarrant" - a white, 28-year-old Australian-born man.
The livestream began as the gunman drove to the Al Noor Mosque in Deans Ave, parking his car in a nearby driveway.
The beige stationwagon contains a cache of weapons and ammunition in the front passenger seat and boot, along with petrol canisters.
He then armed himself and walked into the mosque, with the first victim shot in the doorway.
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Christchurch Hospital is in lockdown.
All appointments have been cancelled this afternoon, and no staff or patients were to enter or leave the building, the district health board said.
Members of the public were told not to go to the hospital unless it was an emergency.
Paramedics are being sent to Christchurch to help.
St John says additional resources are being sent in from around the country.
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A 14-year-old witness to the mosque shooting has described the "running and screaming" that followed.
Idris Khairuddin says prayers were just about to begin when he heard multiple gunshots.
The Hillmorton High School student says his uncle was one of about six people he knew who were shot.
Idris and his uncle are both Malaysians living in Christchurch.
He says his uncle was shot in his backside and he's praying it's not too serious.
Idris says this week was the first time he went to the mosque and is traumatised.
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The Mayor of Christchurch is urging people to lay low and remain calm.
Lianne Dalziel says everyone is in a state of shock.
She says this is not something she would ever expect to see in the city of Christchurch, or New Zealand.
For the latest updates on this developing story, tune in to Newstalk ZB
In other news today, a passionate plea from the country's youth, for our politicians to put the planet up its priority list.
Thousands of students throughout the country have skipped school today, to march and call for action on climate change.
They say it's their future they're concerned about and want adults to listen to their concerns that not enough is being done to protect communities and the environment in future.
Protests got under way in multiple city centres, despite many schools warning that absent students will be marked as truants and could face punishment.
Those in Wellington were carrying banners reading messages such as 'there is no Planet B' and 'change or start swimming'.
Oli Morphew from Wellington Girls' College told the crowd they're sick of hearing about politicians' plans.
She says they need to see action.
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Sporting a swollen black eye, Greens co-leader James Shaw spoke to the thousands of school children protesting at Parliament.
Shaw was assaulted on his way into work yesterday and has a suspected broken eye socket.
And he says it's not going to stop him pursuing the causes he believes in.
Shaw told the striking students to keep up the fight, and keep up the pressure.
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A major review of the police pursuit policy makes no radical changes and the practice will not be banned or pared back in any way.
The joint review by IPCA and police looked at the current pursuits policy.
The review comes at the end of The Chase - a four day Herald series looking into pursuits and the people involved.
Pursuits have increased by 63 per cent between 2011 and 2017.
A sample found the majority of drivers were male - and many had numerous previous convictions.
IPCA has made eight recommendations including improving training and accuracy in communications.
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That's the Front Page for today, Friday, March 15, 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.
If you like to stay up to date on social media, you can find host Juliette Sivertsen on Twitter.
