(Wellington) A white supremacist who killed 51 people at two mosques in New Zealand last year watched without emotion on Monday as relatives of his victims recounted the horror of a massacre which prosecutors said he carefully planned to cause maximum carnage.

Australian national Brenton Tarrant, 29, has pleaded guilty to 51 murders, 40 attempted murders and one charge of committing a terrorist act during the shooting rampage in the city of Christchurch which he livestreamed on Facebook.

He could be the first person in New Zealand to receive a term of life in prison without parole, when a High Court judge sentences him later this week for carrying out the deadliest shooting in the country's history on March 15 last year.

Dressed in grey prison clothes, Tarrant looked at those delivering victim impact statements including the mother of Ata Elayyan, the 33-year-old goalkeeper for the New Zealand futsal team who was slain in the Al Noor mosque.

Maysoon Salama said she constantly wondered what her son was thinking in his last moments "armed only with his courage".

"I can't forgive you ... you gave yourself the authority to take the souls of 51 people. Our only crime in your eyes is that we are Muslims,” she said.

“You killed your own humanity and I don’t think the world will forgive you for your horrible crime. May you get the severest punishment for your evil act in this life, and hereafter.”

Elayyan was near the back of the mosque while his father, near the front, survived the attack despite being shot in the head and shoulder.

Gamal Fouda, imam of Al Noor mosque, told Tarrant that he was “misguided and misled”.

“I can say to the family of the terrorist that they have lost a son and we have lost many from our community too,” Fouda said.

“I respect them because they are suffering as we are.”

Tarrant will be allowed to speak at some point during the hearings, although Justice Cameron Mander has powers to ensure the High Court is not used as a platform for extremist ideology.