Supreme Court judge Sir William Young will lead the Royal Commission of Inquiry into the Christchurch mosque terror attacks.

The Royal Commission will also look at which areas of surveillance the country's spy agencies were putting resources into.

The details were announced today, along with the full terms of reference for the inquiry, by Prime Minister Jacinda Ardern at her weekly post-Cabinet press conference.

"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 told reporters.

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"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."

The Commission will look at:

The alleged gunman's activities before the attack, including:

• Relevant information from his time in Australia.

• His arrival and residence in New Zealand.

• His travel within New Zealand and internationally.

• How he obtained a gun licence, weapons and ammunition.

• His use of social media and other online media.

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• His connections with others in New Zealand and internationally.

• What relevant state sector agencies knew about him and his activities before this attack; what actions, if any, they took in light of that knowledge; and whether there were any additional measures the agencies could have taken to prevent the attack.

• Whether there were any impediments to relevant state sector agencies gathering or sharing information relevant to the attack, or acting upon such information, including legislative impediments.

• Whether there was any inappropriate concentration or priority setting of counter-terrorism resources by relevant state sector agencies prior to the attack.

"Justice Young, who is a sitting Judge of the Supreme Court of New Zealand, New Zealand's highest court, has the extensive experience and skills required to lead the inquiry," Ardern said.

"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."

Ardern said the fact that Young was a Supreme Court judge would help him navigate the inquiry while an active court case was under way.

One further member will be appointed to the commission, which has been allocated a budget of $8.2 million, by the end of April.

It will also engage with the Muslim community, and people will be appointed to help do that.

The inquiry is expected to begin considering evidence from May 13 and report back to Governor-General Dame Patsy Reddy by December 10.

Ardern announced late last month that a Royal Commission would be set up to investigate how the March 15 attack in which 50 people were killed was able to happen.

The SIS, GCSB, Police, Customs, Immigration and any other relevant agencies will come under scrutiny.

Ardern said questions had been raised about spy agencies surveiling only one area prior to the attack and the terms of reference would address that question.

While the inquiry will look at events leading up to the attack, it will not look at the immediate response.

Ardern said the work of first responders would be looked at in a separate review.

Changes to firearms laws are being rushed through following the shootings. The bill was introduced last week and the finance and expenditure committee is expected to report back later today following a truncated submission process.

Ardern vowed to clear the country of military-style semi-automatic weapons and large capacity magazines following the attacks.

Ardern said details of a gun buyback already announced by the Government were still being worked through but she said it was difficult to know how much it would cost because it was not known how many guns were in circulation.