A failure by police to follow their own administrative guidelines allowed the March 15 terrorist to get his gun licence, according to an investigation by leading members of New Zealand's Muslim community.
The Federation of Islamic Associations of New Zealand (FIANZ) wrote a report for the Royal Commission into the attack.
It found the terrorist should never have got the gun licence because he did not have appropriate referees - but police gave it to him anyway.
Mahrukh Sarwar and Nour Malak investigated how police let the terrorist get a gun licence that allowed him to buy the weapons he used in last year's attack.
"If the police had followed their own processes, we are saying they should not have given him the licence," Sarwar said.
The police forms show one referee must be a spouse, partner, or next-of-kin who normally resides with or is related to you, and the other must be a person who is unrelated to you, over 20 years old, and knows you well.
But the terrorist's referees were his online gaming friend and the online gaming friend's father.
The young Muslims say this was an administrative failure by police that had a huge cost.
"If they followed through their own policies that they set, 51 lives would have gone home on that day [on March 15]," Malak said.
The type of gun used in the attack was subsequently banned, but the report shows when the terrorist got his license, police had "deprioritised" checks.
"Due to increased demand in other police priority areas, fewer resources have been available for firearms licensing activities in the 2017-18 year," it says.
Both Sarwar and Malak believe the terrorist should never have been able to get a gun licence.
Haris Murtaza investigated the role of New Zealand's spy agencies and found they wrongly concentrated on Muslim extremists.
He said they were "100 percent" looking the wrong way.
"And because they were putting all their resources and all their focus on the Muslim community, they missed right-wing extremism and they missed the Christchurch terrorist."
The report details that, starting with Anders Breivik's mass murder in Norway in 2011, there have been 45 white supremacist terrorist attacks that should have been warning signs for our spies.
Missed warnings that came at a devastating cost.
"Well it has cost 51 lives, dismantled 100 more, and it's devastated our entire Muslim community," Murtaza said.
Prayers now that it never happens again.