The Christchurch mosque attacks were live-streamed on the internet by a man posting online under the name Brenton Tarrant.
Distressing footage shows him firing indiscriminately at men, women and children at close range inside the Al Noor mosque.
The individual previously posted a rambling and expletive-filled document, espousing violent right-wing ideology.
A man in his late 20s has been charged with murder.
On Friday, Australian Prime Minister Scott Morrison confirmed that a man in custody in New Zealand was an Australian-born citizen.
Mr Morrison called him an “extremist right-wing violent terrorist”, adding that Australian authorities would assist New Zealand’s investigation.
Attack streamed live
The man in the footage equipped himself with what appears to be a head-mounted camera to live-stream the attack in central Christchurch.
The streams were broadcast online, including briefly on Facebook, showing the violence in graphic detail.
A song which played in the suspect’s car is known as a marching anthem for Serbian nationalist paramilitary units known as Chetniks during the 1992-95 Bosnian war.
It praises Bosnian Serb leader Radovan Karadzic, who was convicted of genocide and war crimes.
The names of men convicted of killing Muslims and migrants are written on the suspect’s weaponry.
One item had the words “For Rotherham” written on it, a reference to a child abuse scandal in the UK, while other wording referenced historical battles between European countries and the Ottoman Empire.
Australian media reported that Brenton Tarrant was originally from Grafton, a town 600km (370 miles) north of Sydney, and had previously worked at a fitness facility.
“He never showed any extremist views or any crazy behaviour,” his former boss, Tracey Gray, told Seven News.
The 16,500-word document is called “The Great Replacement”, title of a loose global movement that has been rapidly growing online.
In it, the man says he began planning an attack after visiting Europe in 2017 and being angered by events there.
Specifically, he references a lorry attack carried out by an Islamic State sympathiser in Sweden France’s decision to elect the moderate Emmanuel Macron as president, and ethnic diversity in France.
Despite insisting that he was not motivated by fame, he painstakingly details his thoughts on numerous, unrelated topics. He also acknowledges that he intended to survive the attack, and hoped it would spread fear.
He chose the Al Noor mosque as his target three months ago, the document says.
But investigative website Bellingcat reports that the document was a trap “laid for journalists”, saying it was a form of trolling designed “to derail productive discussion and distract readers”.
The central tenet of the conspiracy is that “European peoples” are dying out and being “replaced” by immigrants with a different, inferior and dangerous culture, says the BBC’s Dominic Casciani.
This is basically a code for hatred or fear of Muslims – part of the theory is that states and corporations are encouraging “white genocide” by pushing up immigration rates purely to keep global capitalism going, says our correspondent.
What about the others in custody?
Prime Minister Jacinda Ardern said none of those in custody had been on active security watch lists.
“Of course the police are currently questioning those in custody, so beyond that there is not too much I can say at this time,” she told reporters on Friday.
She said that “there was a degree of planning” around the attacks, but a full picture was yet to emerge.
Police said the 28-year-old man charged with murder was scheduled to appear in court on Saturday.