New Zealand's royal commission into the terrorist attack against two Christchurch mosques, in which 50 people were killed last month, will report back to the Government by December 10, according to Prime Minister Jacinda Ardern.

Key points: Prime Minister Jacinda Ardern announced the Royal Commission 10 days after the deadly attack

Prime Minister Jacinda Ardern announced the Royal Commission 10 days after the deadly attack It will investigate the of social media, accessibility of guns and whether intelligence agency could have prevented the attack

It will investigate the of social media, accessibility of guns and whether intelligence agency could have prevented the attack The suspect will next appear in court in June

The $7.7 million inquiry will look into the gunman's activities, use of social media and international connections, as well as whether counter-terrorism resources had "inappropriate" priorities, Ms Ardern said in a statement.

Sitting Supreme Court justice Sir William Young will head the investigation, which will also examine the actions of security agencies in the lead-up to the attack, and how Brenton Harrison Tarrant obtained a gun licence and weapons.

The royal commission was announced 10 days after Tarrant used assault rifles and military-style semi-automatics to shoot and kill 50 people who were attending prayer services, injuring dozens more.

Tarrant live-streamed the attack on Facebook and posted a "manifesto" full of anti-immigration and anti-Muslim sentiments on social media and to several politicians, including Ms Ardern, just before the attack began.

When announcing the commission, Ms Ardern said there were many questions that needed to be answered about accessibility of semi-automatic weapons and whether intelligence and security services could have done more to prevent the terrorist attack.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 5 minutes 22 seconds 5 m Thousands gathered for the first Friday prayers in Christchurch since the attack

"It is important that no stone is left unturned to get to the bottom of how this act of terrorism occurred and what, if any, opportunities we had to stop it," she said.

"The royal commission plays a critical role in our ongoing response to fully understand what happened in the lead-up to the attack and to ensure such an attack never happens again.

"I am confident that in his nearly nine years as a judge on our highest bench, Justice Young has the judgement, clarity and care to do the job, with a sound understanding of intelligence issues and experience working in the public eye."

Government moves swiftly on gun laws

The New Zealand Government has already confirmed that it will ban the sale of military-style semi-automatic weapons and assault rifles, which comes in effect on April 11.

Tarrant has been charged with 50 counts of murder over the Christchurch shootings on March 15 and will next appear in court in June.

The Australian shooter had not been on any watch lists in New Zealand or Australia.

Critics have alleged New Zealand's security agencies focused on possible Islamic extremism but did not give enough priority to white extremism.

Wires/ABC