The 17-minute footage he shot with a helmet camera was viewed by more than 200,000 people, according to multiple press reports.
The 28-year-old stormed the first Christchurch mosque, Al Noor, around 1:40 p.m., shooting dead 41 people while live streaming the attack on social media.
"There's no excuse for the content from that livestream to be still circulating on social media now", said Lucinda Creighton, a former government minister in Ireland and an advisor to the Counter Extremism Project, which campaigns to remove violent internet content.
The online platforms "say they have their own technologies but we don't know what that is. There is no transparency, and it's obviously not working", she told AFP. He added: "Take some ownership. There were three in the hallway, at the door leading into the mosque, and people inside the mosque", he said.
Experts said the companies could set their detection tools and removal processes to be more aggressive, but YouTube and Facebook have said they want to be careful not to remove sensitive videos that either come from news organizations or have news value.
The New Zealand Department of Internal Affairs said in a statement that the video footage is "likely to be objectionable content under New Zealand law" and that "people who share the video of the shooting today in Christchurch are likely to be committing an offence".
"We urge people to report all instances to us so our systems can block the video from being shared again".
Rasty Turek, CEO of Pex, a video analytics platform that is also working on a tool to identify re-uploaded or stolen content with YouTube has told The Verge that it is almost impossible to stop live streams as they happen since the content is always changing.
YouTube tweeted about the shooting video, "Our hearts are broken over today's bad tragedy in New Zealand. We are working to have any footage removed". Authorities made Facebook aware of the video shortly after the video began, according to Mia Garlick, Facebook's director of policy for Australia and New Zealand.
Also in a social media post just before the attack, an account that is believed to belong to one of the attackers posted a link to an 87-page manifesto that was filled with anti-immigrant, anti-Muslim ideas and explanations for an attack. He said: "When she heard the noise she wanted to go and make sure her husband was safe". Tarrant also used a message board on 8chan to announce his plan to stream the killings live on Facebook.
Grygiel said it has become commonplace for perpetrators to use social media to gain attention to acts of violence, and that these are often shared on YouTube and other platforms.
Alex Zhukov, founder and chief technology officer of LIVE4 developer VideoGorillas, said the LIVE4 services transmitted footage directly to Facebook and his company did not have the ability to review it first.