A man aged in his 20s has been charged with murder following the shootings at the al Noor mosque in central Christchurch and then the nearby Linwood mosque.
Three other people – two men and a woman – were also arrested after the attacks.
At least 49 people have been killed in mass shootings at two mosques full of worshippers attending Friday prayers in the New Zealand city of Christchurch.
Prime Minister Jacinda Ardern said it was “one of New Zealand’s darkest days” and called the shootings a terrorist attack.
Another 20 people were seriously injured, she said.
One man who was inside the al Noor mosque – the scene of the deadliest shooting – told local media that there would normally be dozens of people worshipping inside.
The witness, whose clothes had been splattered with blood, said the shooting lasted for about 20 minutes and said the gunman methodically went from room to room.
Australia’s prime minister Scott Morrison confirmed one of the suspects was an Australian-born citizen.
The gunman filmed himself in his car before he carried out the attacks and also live-streamed the atrocity.
Ms Ardern said the country had been placed on its highest security threat level.
A man who claimed he was behind one of the attacks left a 74-page anti-immigrant manifesto.
At a news conference, Police Commissioner Mike Bush said a number of schools had been placed on lockdown, as well as the two mosques that were targeted.