This is the first picture of one of the suspected gunmen who stormed a mosque and opened fire on worshippers.
Footage posted on the Facebook page of a man called Brenton Tarrant was live streamed showing the massacre unfolding.
At least 40 people have been killed in the horror shootings which appear to have been inspired by right-wing ideology.
Police said multiple fatalities had occurred at two mosques, but it was unclear how many attackers were involved.
Footage from one mosque, which has circulated on social media, was posted online live as the attack unfolded.
In the killer’s live-stream, he can be seen parking his vehicle outside the Hagley Park mosque, before taking two guns and walking a short distance to the entrance of the building.
The gunman says “let’s get this party started” as he drives up with loud music playing.
He also says “Remember lads, subscribe to pewdiepie”, referencing the controversial YouTuber.
He then opens fire. Over the course of five minutes, he repeatedly shoots worshippers, leaving well over a dozen bodies in one room alone.
He returned to the car during that period to change guns, before going back to the mosque to shoot anyone showing signs of life.
The man then walks back outside and shoots a young girl on the pavement as she screams for someone to help.
The authenticity of the footage – which has since been taken down – has not been verified.
Police said the second mosque attacked was in the suburb of Linwood.
Three men and one woman are in custody following the shootings at Masjid Al Noor Mosque and Linwood Masjid Mosque in Christchurch at around 1.40pm local time.
One of those arrested was wearing a suicide vest, cops confirmed as they warned “let’s not presume the danger has gone”.
Following the shootings, New Zealand Prime Minister Jacinda Ardern said it is “one of New Zealand’s darkest days”.
In a press conference, Police Commissioner Mike Bush warned: “We’re not assuming this is contained to Christchurch.”
One of four people arrested is Australian, Australian Prime Minister Scott Morrison said on Friday.
“I can confirm that the individual who was taken into custody I have been advised is an Australian-born citizen,” he told reporters in Sydney.
“As family members with our New Zealand cousins today, we grieve, we are shocked, we are appalled, we are outraged, and we stand here and condemn absolutely the attack that occurred today by an extremist, right-wing, violent terrorist.”