An armed police officer stands guard outside the Christchurch High Court during the sentencing hearing for Brenton Tarrant, the gunman who killed 51 people at two New Zealand mosques last year

CHRISTCHURCH — New Zealand mosque gunman Brenton Tarrant was sentenced to life in prison without parole on Thursday for the massacre of 51 Muslim worshippers, with a judge calling him "wicked" and "inhuman".

As the sentence was read out there was jubilation outside the courtroom, with crowds cheering and singing the national anthem – "God Defend New Zealand".

Judge Cameron Mander said Tarrant's "warped" ideology and "base hatred" led the Australian white supremacist to murder defenceless men, women and children last year in New Zealand's worst terror attack.

"Your crimes are so wicked, that even if you are detained until you die it will not exhaust the requirements of punishment and denunciation," Mander said as he announced a sentence that is a New Zealand legal first.

The judge solemnly read out the names of those Tarrant executed in his livestreamed rampage and recounted in forensic detail how he shot the wounded as they pleaded for help on March 15 last year.

"It was brutal and beyond callous. Your actions were inhuman," he said, pointing out that Tarrant deliberately attacked Friday prayers to maximise casualties.

Tarrant, 29 – who waived his right to speak at the hearing – retained the same impassive demeanour through the four-day hearing as survivors and bereaved family members gave heart-wrenching testimony of their incalculable loss.

Gamal Fouda, the imam of Al Noor mosque – one of those targeted by Tarrant – said the sentence was what the Muslim community had hoped for.

"But no punishment is going to bring our loved ones back and our sadness will continue for the rest of our lives," he said.

Prime Minister Jacinda Ardern, who was praised for her compassionate and decisive response to the shootings, also welcomed the sentence.

"The trauma of March 15 is not easily healed but today I hope is the last where we have any cause to hear or utter the name of the terrorist behind it," she said.

"He deserves to be a lifetime of complete and utter silence."

She also expressed hope that members of the country's traumatised Muslim community feel "the arms of New Zealand around you".

Tarrant sparked global revulsion when he rampaged through two Christchurch mosques for 20 minutes during Friday prayers.

He had admitted 51 charges of murder, 40 of attempted murder and one of terrorism over the attacks, after reversing an initial not-guilty plea.

‘Racist and xenophobic’

Crown prosecutor Mark Zarifeh said the atrocity was "without comparison in New Zealand's criminal history".

"The offending was motivated by an entrenched racist and xenophobic ideology... in my submission, the offender is clearly New Zealand's worst murderer," he said.

Zarifeh said life behind bars was "the only proper sentencing option" for Tarrant.

"No minimum period is sufficiently long to satisfy sentencing objectives given the gravity of the offending and the devastating loss of life and injury," he said.

For much of the four-day sentencing, the court heard testimony from dozens of his victims and their families.

"Since my husband and son passed away, I've never had a proper, normal sleep. I don't think I ever will," widow Ambreen Naeem told the court.

"His punishment should continue forever," she said.

Fearing Tarrant may use the platform to spout extremist ideology, the court had imposed tight restrictions on the reporting of proceedings.

Before the sentencing, Tarrant, a former gym instructor, had sacked his legal team and declared he would represent himself.

Instead, court-appointed lawyer Pip Hall made a brief one-line statement on his behalf before the judge delivered his sentence.

"Mr Tarrant does not oppose the application that he should be sentenced to life imprisonment without parole," Hall said.

Arguing against life behind bars, counsel assisting the court Kerry Cook said Tarrant's views had changed while he had been jailed and he had offered to meet the families in a "restorative justice" session.

"Given his age, lack of previous record and guilty pleas, there is a prospect of rehabilitation," he told the court, saying a whole-life sentence breached fundamental human rights.

But Zarifeh said Tarrant's belated description of his actions as "unnecessary, abhorrent and irrational" were questionable.

"(Tarrant) said he had a poisoned emotional state and was terribly unhappy," Zarifeh said.

"He felt ostracised by society and wanted to damage society as an act of revenge.

"Yet at the same time, the offender described the offending as definitely an act of terrorism."

The atrocity shocked New Zealand and prompted Ardern to immediately tighten gun laws and pressure social media giants to curb online extremism. — AFP