"My heart and thoughts go out to the victims, families and everyone affected by this tragedy".
The perpetrator wore a body camera and appears to have live-streamed the attack on Facebook. The shooter also released a 74-page manifesto, titled "The Great Replacement", online before the shooting.
He said he wanted to "take revenge" for Europeans lost to terror attacks in Europe, and it appears that he was most outraged by the death of Ebba Akerlund, a Stockholm schoolgirl who was one of the four people killed by a Muslim asylum-seeker who drove a hijacked lorry into a crowd in April 2017.
Nearly 50 people were treated at Christchurch Hospital for gunshot wounds, including young children.
Unlike in the United States, where calls for greater gun control after mass shootings are often met by deaf ears when it comes to federal regulation, three mass shootings - only one of which happened in New Zealand - prompted legislative changes. The suspect will appear in the Christchurch court on March 16.
Ranstorp said Tarrant's document is shorter and "more sloppy" than the one written by the Breivik, who killed 77 people in 2011.
"He very much wished for his condolences to be passed on to New Zealand", said Ardern.
Police have confirmed they are responding to a "critical incident".
The suspects' ideology and preparation for the attack - Prime Minister Ardern described them as having "extremist views", but New Zealand's police commissioner said the man charged with murder was not known to intelligence services. As New Zealand has stood by us so we stand shoulder to shoulder with them, and with Muslims in New Zealand, here in the United Kingdom and around the world. The UK stands ready to support New Zealand however we can.