A long gunman killed 51 people at two mosques on March 15 while livestreaming the massacre on Facebook. It was New Zealand's worst peace time shooting.

Australian Brenton Tarrant, a suspected white supremacist, has been charged with multiple counts of murder for the mass shooting.

New Zealand's Royal Commission inquiry will look into Tarrant's alleged activities, use of social media and international connections, as well as whether there was inappropriate priority settings in counter terrorism resources.

"The Commission's findings will help to ensure such an attack never happens here again," Ardern said in a statement on Monday announcing a second commissioner to the inquiry.

The Commission will report its findings to the government on December 10.

Ardern is in Paris this week to co-chair a meeting with French President Emmanuel Macron that seeks to have world leaders and chiefs of tech companies sign the "Christchurch Call", a pledge to eliminate terrorist and violent extremist content online.

In an opinion piece in The New York Times on Saturday, Ardern said the "Christchurch Call" will be a voluntary framework.

"This is not about undermining or limiting freedom of speech. It is about these companies and how they operate," Ardern said her column.

Representatives from Facebook, Google, Twitter and other tech companies are expected to be part of the meeting, although Facebook head Mark Zuckerberg will not be in attendance.

Facebook said Nick Clegg, the former deputy prime minister of the UK, and currently Facebook's Vice President for Global Affairs and Communications, will attend the meeting.

"These are complex issues and we are committed to working with world leaders, governments, industry and safety experts at this week's meeting and beyond on a clear framework of rules to help keep people safe from harm," Klegg said in a statement emailed to Reuters.

Australian Associated Press