Top judge named to head Christchurch massacre inquiry

New Zealand Prime Minister Jacinda Ardern on Monday named a Supreme Court judge to lead an inquiry into the massacre last month of 50 worshippers at two Christchurch mosques.

"The government will ensure no stone is left unturned as we examine as quickly as possible how the March 15 attack happened, what could have been done to stop it and how we can keep New Zealanders safe," Ardern said.

The Royal Commission probe, the most powerful judicial inquiry available under New Zealand law, will be led by Supreme Court Judge William Young and report its findings by December 10, she said.

Australian Brenton Tarrant, 28, a self-avowed white supremacist, has been charged with 50 counts of murder and 39 of attempted murder over the mosque attacks in the southern city.

Ardern said the probe, due to begin hearing evidence on May 13, would play a critical role in ensuring "such an attack never happens again".

The inquiry will involve consultation with the Muslim community and look into whether New Zealand counter-terror agencies placed an "inappropriate concentration" of resources in tracking Islamist militants while neglecting the threat from far-right extremism.

It will probe Tarrant's activities before the attack, including how he obtained a gun licence, weapons and ammunition, and his use of social media, Ardern said.

Tarrant live-streamed his attack and published a lengthy manifesto online before launching his rampage.

What knowledge intelligence agencies had of his activities and whether any laws impeded the gathering and sharing of information by the agencies will also be part of the inquiry, she said.

Since the attacks, the government has tightened the country's gun laws, is reviewing legislation dealing with hate speech and called for social media giants to do more to combat online extremism.