Extremists around the world have sought to emulate Breivik ever since his deadly attacks in Norway which left 77 people dead in 2011. The Christchurch attacks bore several of the features of Breivik's: mass shootings, multicultural victims, a racist manifesto published online and inscribed weapons.
In a 74-page document posted on Twitter just before the attack, the Christchurch shooter said he "took true inspiration from Knight Justiciar Breivik", using terminology reminiscent of that used by the Norwegian extremist.
"I have only had brief contact with Knight Justiciar Breivik, receiving a blessing for my mission after contacting his brother knights," wrote the shooter.
A lawyer for Breivik, Oystein Storrvik, told Verdens Gang newspaper that "it seems unlikely" the Christchurch attacker had been in direct contact with Breivik, given the strict controls imposed on him in prison.
Breivik killed 77 people on July 22, 2011 when he set off a van bomb near government offices in Oslo, then opened fire on a Labour youth camp on the island of Utoya.
He said he killed his victims because they embraced multiculturalism. The now 40-year-old Norwegian had also posted a more than 1,500-page manifesto in which he called on others to follow his example.
Australian Prime Minister Scott Morrison described the Christchurch attacker as an Australian "extremist, right-wing, violent terrorist".
For Norway, the Christchurch attacks brought back memories of the Breivik attacks, its most violent event since World War II.
"It recalls painful memories," Prime Minister Erna Solberg said. "Anyone who has ever lost a loved one, whether it's because of terrorism or not, understands what these families are going to go through," Vanessa Svebakk, a Norwegian who also holds New Zealand citizenship and who lost her 14-year-old daughter in the Utoya attack, told AFP.
"But for those of us who have lost someone because of terrorism, the feelings are even stronger."
Tore Bjorgo, the head of the University of Oslo's Center for Extremism Research, said "there are clearly a lot of the same ideas behind" the two attacks.
They include, among other things, "the idea that European civilisation is threatened by immigration in general and by Muslim immigration in particular, and that it is legitimate for some people to resort to extreme violence to stop it," he told AFP.
"There are pretty clear indications in the (Christchurch) manifesto that we're dealing with a white supremacist," said Jean-Yves Camus, a French expert on far-right movements.
"The manifesto goes further than what Breivik wrote in his own text. Breivik didn't describe himself as fascist," he said.
Like Breivik, the Christchurch killer compared himself in his manifesto to Nelson Mandela, saying he even expected to win the Nobel Peace Prize one day.
Both attackers share "this narcissism, this grandiose image of themselves," Swedish terrorism researcher Magnus Ranstorp told AFP.
Breivik, who now goes by the name Fjotolf Hansen, is serving a 21-year-sentence that can be extended indefinitely. He is held in isolation without internet access, and his limited contacts with the outside world are closely monitored, at times blocked.
"If the correspondence could inspire acts of violence, then we are allowed to stop it," Espen Jambak, the deputy head of the high security Skien prison where Breivik is incarcerated, told AFP.
"We feel we have good control" over his correspondence, he said.
Breivik's attacks have already inspired other extremists in the past. On July 22, 2016, exactly five years after the Norway attack, a young man with mental health issues and said to be obsessed with Breivik killed nine people in a Munich shopping centre before committing suicide.
"There have also been other terror plots inspired by Breivik at more or less advanced stages, in Poland, the Czech Republic, France and the United States," said researcher Bjorgo.
Each attack risks triggering new ones.
The New Zealand attack "was clearly devised to inspire others, both those on the extreme right and Islamist extremists," Utoya survivor Bjorn Ihler told AFP.
"That it was filmed live indicates there was a deliberate strategy to create a narrative that can be used by extremists on both sides."