In total, he murdered 51 men, women and children and wounded 40 others.
The commission's findings were made public yesterday and it was established that other than the email sent by Australian national Tarrant to the Parliamentary Service just eight minutes before he opened fire at the Al Noor Mosque – there was no other information provided or otherwise available to any relevant public sector agency that "could or should have alerted them to the terrorist attack".
Among the information provided in the 792-page Royal Commission report - which includes 44 recommendations the Government has agreed in principle to adopt - more detail was released about Tarrant and his background.
The inquiry concluded that Tarrant is appropriately labelled as "a lone actor".
"There is no evidence that anyone else was aware of his plans or provided personalised encouragement," the commission found.
"It is, however, likely that his thinking was affected by what was said in far-right online communities and other far-right material he was about to source from the internet."
The inquiry, which interviewed Tarrant behind bars, delves deep into his background to try and understand what influenced his extreme views, why he chose New Zealand for a terrorist attack, and how he avoided coming to the attention of authorities.
While the report named him in the first instance, he is referred throughout as "the individual".
He displayed racist behaviour from a young age, the inquiry found, primarily concerned about immigration and Muslims moving to Western countries. He was an avid internet user and online gamer, with few childhood friends.
"His life experiences appear to have fuelled resentment and he became radicalised, forming extreme right-wing views about people he considered a threat. Eventually, he mobilised to violence," the report says.
He used the A$457,000 given to him by his father Rodney before he died by suicide in April 2010, to travel extensively, firstly exploring New Zealand and Australia in 2013, and then around the world between 2014 and 2017.
He visited dozens of countries, including North Korea, China, Russia, Georgia, Turkey, Israel, India, Serbia and Indonesia, as well as many other African, Asian, European and South American nations.
There was no evidence, however, that he met with any extremist groups while oversees, investigated potential targets, or carried out any training.
The inquiry did not believe that the travel fuelled his racist views to any great extent, rather concluding: "Put simply, he travelled widely because he could and had nothing better to do."
But he did visit right-wing internet forums, subscribe to right-wing YouTube channels and "read a great deal about immigration, far-right political theories and historical struggles between Christianity and Islam".
He was a follower and prominent commenter on the Facebook pages of the United Patriots Front and The True Blue Crew far-right groups based in Australia.
He also visited controversial online bulletin boards 4chan and 8chan.
A copy of the Oslo terrorist's manifesto was later found on the gunman's SD memory card in a drone he used to fly over the Al Noor Mosque for pre-attack reconnaissance. The inquiry found the Oslo terrorist to have been a significant influence on his thinking.
The inquiry was sure that Tarrant came to New Zealand in August 2017 with a terrorist attack in mind.
Eight months earlier, he had emailed the Bruce Rifle Club near Dunedin asking if it was still operating.
And just 15 days after landing in New Zealand, he took his first step towards obtaining a firearms licence by paying the application fee.
Four days later, he undertook and passed the required Firearms Safety Course.
He was then required to provide two referees – one of whom needed to be a near relative.
The terrorist identified his sister Lauren Tarrant and a gaming friend who he'd played online games with over a decade but had only been physically in the company of for about 21 days over that entire period.
But New Zealand Police did not accept his sister as a referee because she was in Australia and could be spoken to in person.
In the end, the gaming friend's parent was added as a referee.
The licence application was approved on November 16, 2017, and on December 4, 2017, Tarrant - deemed a fit and proper person to handle firearms - purchased his first gun.
During the Royal Commission, it emerged that Tarrant's gaming friend was "well aware" of his extremist political opinions and that he was racist and Islamophobic.
But they did not tell the vetting officer.
In the lead up to March 15, Tarrant - a former personal trainer and gym enthusiast - used steroids to bulk up and was solely focused on planning and preparing his attack.
The inquiry found he was isolated and avoided social situations.
"For the more than 18 months he lived in New Zealand preparing for the terrorist attack, he remained resolutely focused, attempting to maintain operational security from which there were only limited lapses," the report notes.
While living in New Zealand, he made at least 14 donations to far-right and anti-immigration groups and individuals, including AU$2,308.97 directly to Identitarian Movement Austria's leader Martin Sellner.
Tarrant joined the Otago Shooting Sports Rifle and Pistol Club in February 2018 but only visited twice after becoming a member of Bruce Rifle Club where he shot more than 27 times.
While generally described by gun club members as polite and courteous, not talking much, if at all, about politics.
But a few members raised aspects of his behaviour, which with hindsight, may be significant, including the fact he usually shot while standing up and went through a large amount of ammunition. His primary interests appeared to be firing at extremely fast rates and changing magazines quickly, the told the inquiry.
And on July 13, 2018, he had a close shave.
While cleaning a rifle barrel in his lounge, a round of ammunition exploded, sending fragments into his right eye and thigh, requiring treatment at Dunedin Hospital's emergency department.
He was discharged later that day and although worried that the incident might draw attention, hospital staff did not tell police.
On the night of the deadly attacks, he phoned his mother across the Tasman and they spoke for 28 minutes. She later told police he seemed relaxed and happy but made a point of telling her that he loved her, which was out of character. He also spoke for over an hour with his sister – also saying he loved her.