A criminology professor says the life without parole sentence of terrorist Brenton Harrison Tarrant was the only one that could have been handed down.

The sentence given to the Christchurch gunman by Justice Mander is the first one to be delivered in New Zealand.

The gunman, who is 29, was sentenced at the Christchurch High Court of 51 counts of murder, 40 of attempted murder and a charge of committing a terrorist act. He had previously pleaded guilty to all the charges.

It is the first time in New Zealand that someone has been sentenced under the Terrorism Suppression Act 2002.

John Pratt, who is a Professor of Criminology at the Institute of Criminology at Victoria University of Wellington, says the Judge had little option but to impose life without parole.

“It would’ve been possible to impose a sentence of say 30 years without parole but he has murdered 51 people, and injured 40 others in a carefully planned attack. That attack has dwarfed anything we had had in this country before, so the sentence reflects those crimes.”

Professor Pratt says the sentence doesn’t set a precedent for sentencing in the future as “there is nothing to match the appalling nature of this incident, which you would hope was a one-off.”

He also said he was impressed by the numerous victim impact statements and that those statements have brought home the gravity of the incident.

RNZ News reports that Justice Mander told the killer his ideology of hate was “anathema to the values of New Zealand’s society”

“Even if you are detained until you die, it would not exhaust the requirements of punishment and denunciation," he said.

On the first day of sentencing, New Zealand Law Society | Te Kāhui Ture o Aotearoa president Tiana Epati paid tribute to those providing victim impact statements.

“This will be a testing week for all involved and my aroha goes out to the victims and their families”, she said.

She went on to call for public understanding of the lawyers acting as independent counsel and standby counsel.

“Despite the extreme nature of this case justice must be served. This can only happen when there is competent and experienced counsel on both sides. The Law Society stands behind all the lawyers involved in this immensely challenging case.”