The Christchurch Call summit in Paris will be the culmination of weeks of effort from Prime Minister Jacinda Ardern, underpinned by the warm relationships she nurtured with world leaders last year.

On the eve of the national memorial service two weeks after the March 15 terrorist attack, Jacinda Ardern had a phone call with German Chancellor Angela Merkel to discuss how to curb social media harm.

The New Zealand Prime Minister had met Merkel in Germany last year on a trip seeking support for a New Zealand-European Union free trade deal, an encounter Merkel described in jovial terms : "Time flew and it was nice, it was very interesting and fun."

Merkel has long maintained excellent relationships with New Zealand, including with former Prime Minister John Key. She visited New Zealand in 2014, a trip she fondly recalls for releasing a kiwi on Motutapu Island.

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Ardern was looking for a way to lead a global initiative to pressure social media companies into taking more responsibility for the content on their platforms.

As a possible launching pad, she and Merkel discussed the "Tech for Humanity" meeting of G7 Digital Ministers and France's "Tech for Good" summit, both in Paris on May 15.

The next evening, after the memorial service and two-minute silence that brought the country to a standstill, Ardern called French President Emmanuel Macron, who had warmly welcomed her to Paris last year just before she headed to Germany to meet Merkel.

"It became clear from my conversation with Macron that there was a platform for us to bring people together," Ardern told the Herald .

"They [the French] were really supportive. They've had their own experiences. From there, we collaborated to generate the Christchurch Call."

Ardern was reluctant to take credit for a summit that she and Macron are co-chairing, telling the Herald that "New Zealand initiated it".

PM Jacinda Ardern talks ahead of the Paris summit

There were many pressing issues for Ardern following the March 15 attack, including support for the victims and their families, immediate gun law reform, setting up an inquiry into how the attack happened and whether it could have been prevented, and planning a memorial service.

But the Herald was told that Ardern had social media harm firmly on her radar as well, and discussed it at the Odesc - Officials Committee for Domestic and External Security Coordination - meetings immediately following the attack.

The focus was the way in which the gunman live-streamed the attack on Facebook, and the way it went viral; the video was uploaded 1.5 million times in the first 24 hours.

In an opinion piece published in the New York Times ahead of the summit, Ardern disclosed publicly for the first time that she inadvertently watched the video footage of the Christchurch terror attack.

"I use and manage my social media just like anyone else. I know the reach of this video was vast, because I too inadvertently saw it," she said.

On March 18, three days after the attack, Ardern said during her post-Cabinet press conference: "I would call on our social media platforms of all variety to demonstrate the kind of responsibility that both lead to these events, and that includes those who perpetuate the messages in the aftermath. There's a lot of work that needs to be done."

The following day in Parliament, the first sitting day since the shootings, Ardern turned up the rhetoric: "We cannot simply sit back and accept that these platforms just exist and that what is said on them is not the responsibility of the place where they are published. They are the publisher, not just the postman. There cannot be a case of all profit, no responsibility."

On March 25, she met Microsoft president Brad Smith at the Beehive along with Ministers David Parker, Andrew Little and Kris Faafoi. Smith is understood to be very supportive of a global initiative, and is expected to be at the summit in Paris on Thursday (NZT).

One of the issues that is expected to be traversed at the summit is the AI technology that could be used to block such content as it is uploaded.

Facebook has said its algorithms are not advanced enough at the moment, and the live-streamed video from March 15 was not gruesome enough for the AI to detect.

A Facebook technician reportedly said this in a closed congressional briefing in the US on March 27.

The following day, Ardern called Merkel.

Q&A

• What? The Christchurch Call summit in Paris on Thursday (NZT), chaired by Jacinda Ardern and French President Emmanuel Macron

• Why? Following the terror attack on March 15, the aim is to seek a commitment from country leaders and tech companies to stop terrorist content and violent extremism on social media platforms

• How? Not by reactionary means, such as government regulation, but by preventative means, such as a pledge to develop more advanced technology to block unsuitable content as it is uploaded

• Who? Facebook, Microsoft, Twitter, Google, Amazon representatives are going, as well as seven country leaders including the UK's Theresa May and Canada's Justin Trudeau. No-shows from US President Donald Trump and Facebook boss Mark Zuckerberg.