Helen Clark has accused Facebook of playing host to terrorism, with its failure to stop the live broadcast of the Christchurch attack.
A gunman shot 100 people at two mosques in the city last Friday, killing half of them. The atrocity was streamed on Facebook Live.
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Up to 200 people watched it as it happened, and not a single one of them reported it to Facebook.
"The first user report on the original video came in 29 minutes after the video started, and 12 minutes after the live broadcast ended," said Facebook spokesperson Chris Sonderby. The video was removed "within minutes" of police getting in touch.
Around 1.5 million copies and attempts to upload it were blocked by Facebook in the following 24 hours, but Clark says that's not good enough.
"How can a mass murderer be broadcasting his crime live for 17 minutes and nothing is done? Unbelievable," she told The AM Show on Thursday.
"They have been hosting terror. If you broadcast a set of killings live for 17 minutes, you are hosting terror. You've done nothing to stop it, have you?"
She said while traditional media was regulated through either legislation or industry bodies, there is presently little that can be done to keep social media outlets in line.
"Social media ain't self-regulating. That's when the regulators need to think about, do we come in?"
The suspect wasn't on any terror watchlists before the attack. Clark suggested he may have regularly visited sites that host extremist content, but didn't post much himself.
"He may have kept quite a low profile in terms of what he posted himself until this last mad frenzy... and that's too late. It seems to me that because it's reasonably clear he was networking with other extremists, that there has to be something in the social media algorithms which says that this fellow is obsessively sitting on these sites.
"And by the way, why are these sites allowed? I think some people take the principle of freedom of speech to the extent that anything goes. You can have sites where people discuss who they're going to kill and how they're going to do it, and this goes? I don't accept that."
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The site the suspect posted his manifesto to, 8chan, has been blocked by a number of New Zealand and Australian ISPs.
The minister responsible for New Zealand's intelligence agencies, Andrew Little, says there will be an initial review to get a "quick answer" on why the suspect wasn't being tracked, likely to be followed by a more in-depth inquiry into their failings.
"We need to know how visible he was," said Clark. "I'm really pleased there's an inquiry because everything needs to come out. The public need to know what they're being protected from and what they're not."
She said while Islamic State has been a "very real threat" over the past few years, surveillance may have to be stepped up to fight white supremacists.
"Let's get real - this alt-right extremism is now a significant security issue."
'Alt-right' is an umbrella term for white supremacists, hate groups and conspiracy theorists, many of whom support US President Donald Trump, who have been linked to dozens of fatal attacks in recent years.