He enters the building and fires repeatedly at worshipers as he moves from room to room.
"The content of the video is disturbing and will be harmful for people to see", the department said.
"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.
Internet platforms have cooperated to develop technology that filters child pornography, but have stopped short of joining forces on violent content.
Britain's interior minister, Sajid Javid, said on Twitter, "Enough is enough".
Facebook, Instagram, Twitter, YouTube and Google all use artificial intelligence combined with human monitors to try to stop videos depicting violence from being shared, but it appears none of the platforms initially caught the video that streamed live for 17 minutes.
"Our hearts go out to the victims, their families and the community affected by the horrendous shootings in New Zealand". "It is ever clearer that YouTube, in particular, has yet to grapple with the role it has played in facilitating radicalization."Facebook, Twitter and YouTube all said they were taking action to remove the videos".
New Zealand's Department of Internal Affairs said people posting the video online risked breaking the law. "We would strongly urge that the link not be shared". "We will continue working directly with New Zealand Police as their response and investigation continues".
YouTube, which is owned by Google, tweeted: "Our hearts are broken over today's awful tragedy in New Zealand".
Twitter, YouTube owner Google and Reddit also were working to remove the footage from their sites.
"New Zealand Police alerted us to a video on Facebook shortly after the livestream commenced and we removed both the shooter's Facebook account and the video. We're also removing any praise or support for the crime and the shooter or shooters as soon as we're aware", Facebook said in a statement.
The hours it took to take the violent video and manifesto down are "another major black eye" for social media platforms, said Dan Ives, managing director of Wedbush Securities.
Social networks have been caught flat-footed in many cases by videos showing violent acts including suicides and assassinations.
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.
Meanwhile, tech companies such as Facebook were working to take down the video, Fox News reported.
And Twitter said it removed an account that was said to be linked to the suspect.
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