The Royal Commission into the attack on two Christchurch mosques last year has interviewed the terrorist who killed 51 people.

Brenton Tarrant has been convicted of the murders, along with an additional 40 charges of attempted murder, all relating to the shootings at Masjid Al Noor and Linwood Islamic Centre on March 15.

His shock guilty pleas in March included admitting engaging in a terrorist act.

A Royal Commission is investigating his activities in the lead-up to the mass shootings and this has included interviewing the convicted terrorist in private at Auckland Prison.

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Commissioner Jacqui Caine said it was a "carefully considered decision driven by the need to observe natural justice" and with the ultimate goal of providing answers to the New Zealand public.

"We have given a commitment to the public that we would leave no stone unturned and interviewing the individual is another way we have sought to fulfil that commitment."

Chair of the Royal Commission Justice Sir William Young said while the Royal Commission had undertaken extensive investigations with the benefit of material provided by the New Zealand Police and other agencies, there were some areas of uncertainty.

"In addition, the Terms of Reference and the Inquiries Act 2013 require the Royal Commission to observe the principles of natural justice in respect of any person who may be the subject of adverse comment in the Royal Commission's report.

"That includes the individual."

The terrorist, and his lawyers, as is the case with others who have been interviewed by the Royal Commission, have been made the subject of confidentiality orders under the Inquiries Act 2013 and are not at liberty to discuss the content of the interview.