New Zealand mosque gunman Brenton Tarrant was sentenced to life in prison without parole 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. 

"His 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.

'Poisoned' gunman's road to extremism

There were no obvious warning signs when the Christchurch mosques gunman moved to New Zealand from Australia in 2017 -- he had no criminal history and was not on any security watch list.

But 29-year-old Brenton Tarrant will now go down in history as New Zealand's first convicted terrorist, and the first person in the country ever sentenced to life imprisonment without parole.

Born in the rural Australian town of Grafton, a six-hour drive north of Sydney, Tarrant worked as a gym instructor before arriving in New Zealand.

Only later did it emerge that Tarrant began amassing an arsenal of weapons soon after setting up home in Dunedin with the intention of carrying out an atrocity against New Zealand's Muslim community.

After meticulous preparation, the plan came to a ghastly conclusion on March 15 last year when Tarrant attacked two mosques in Christchurch, livestreaming the event as it happened.

"He intended to instil fear into those he described as 'invaders', including the Muslim population or more generally non-European immigrants," prosecutor Barnaby Hawes told a sentencing hearing at Christchurch High Court this week.

As the world searched for answers, former friends and colleagues were quizzed about Tarrant's background and possible motivations.

Details emerged of a socially awkward loner who became a gym junkie after being bullied as an overweight teenager.

He was also apparently hit hard when his father died of cancer in 2010 at the age of just 49 -- but there was nothing that remotely explained the searing hatred behind Tarrant's crimes.

- 'Act of revenge' -

In a rambling "manifesto" posted before the massacre, Tarrant talked of being radicalised during trips to Europe and Asia, apparently financed by an inheritance that meant he did not have to work.

An exceptional aspect of Tarrant's personality seems to be his susceptibility to online hate and, eventually, his willingness to weaponise the internet to share his killing spree on social media via a helmet-mounted GoPro camera.

Increasingly isolated in the real world, Tarrant dwelt in extremist chat rooms, sharing racist memes and in-jokes with online acquaintances who encouraged his views. 

Prosecutor Mark Zarifeh quoted from an interview prison authorities conducted with Tarrant in April, when he described his state of mind at the time of the attacks.

"He 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."

Minutes before the massacre, Tarrant sent a message to the now-defunct extremist website 8Chan saying it was "time to make a real-life effort post". "You are all top blokes and the best bunch of cobbers (friends) a man could ask for," he wrote.

Scrawled on his weapons were the names of numerous historical military figures -- many of them Europeans involved in the Crusades or in fighting Ottoman forces in the 15th and 16th centuries.

In court, Mirwais Waziri, who survived a bullet to the neck, punctured any self-aggrandising illusions Tarrant may have held about being some sort of racial warrior on a historical mission.

He reminded Tarrant that the youngest fatality in his attack on unarmed men, women and children was three-year-old Mucaad Ibrahim, shot twice while clinging to his father's leg for protection.

"He did not have religion, faith or colour. He didn't know anything about that," Waziri said. "How are you going to answer that... how are you going to face God on judgement day and answer how and why you killed a three-year-old boy?"

Tarrant, despite the bluster contained in his pre-massacre manifesto, could find no words to try to justify himself, or express remorse, and he waived his right to speak at the hearing.

Noticeably thinner than the bulked-up killer who flashed a white-power hand signal at the court the day after the attacks, he remained mute and submissive as jailers led him away to serve his life sentence.