Tarrant, who is due to appear in court on Saturday, identified himself online before live-streaming himself slaying more than 40 people at one of the mosques.
On a since-deleted Twitter account, the Australian posted a series of photos of apparent machine-gun magazines and a link to what is being called a manifesto attempting to justify his actions.
The rambling 74-page document espouses anti-immigrant and Islamophobic ideology, and features tropes familiar to white supremacist and alt-right fringe movements.
He lists the desire to cultivate “an atmosphere of fear” for Muslims among a series of aims and repeatedly refers to violent uprisings against followers of Islam.
Many of the passages refer to news stories involving Muslims repeatedly cited by right-wing commentators.
The author describes himself as an “ordinary white man” of “Scottish, Irish and English stock” who had little interest in education and failed in his exams.
He claims he did not carry out the attack for fame, and that he is a “mostly introverted” person who identifies as an ethno-nationalist and a fascist.
He also expresses support for US president Donald Trump as a “symbol of renewed white identity and common purpose”, but suggests he does not admire him as a “policymaker and leader”. He claims Brexit was a vote against “globalism”.
The document claims he had “brief contact” with Anders Breivik and that the Norwegian mass killer gave his “blessing” for his attack.
According to investigative website Bellingcat, it also features a number of memes and in-jokes aimed at far-right online users who frequent websites such as 4chan, as well as attempts at misdirection to fool journalists and increase division.
Police are yet to confirm Tarrant is in custody over the shooting, but have said one man has already been charged. Scott Morrison, Australian prime minister, confirmed an Australian national was involved in the attack.
ABC News reported Tarrant worked as a personal trainer in New South Wales, Australia, between 2009 and 2011, before he left to go travelling in Asia and Europe.
“He was a very dedicated personal trainer,” a former colleague of Tarrant’s told the news outlet. She added: “I think something must have changed in him during the years he spent travelling overseas.”
She said Tarrant’s father, Rodney, died from an asbestos-related illness when Tarrant was finishing secondary school, and that he has a surviving mother and sister.
“I honestly can’t believe that somebody I have probably had daily dealings with and had shared conversations and interacted with would be capable of something to this extreme,” she said.