The sentencing of a gunman who killed 51 Muslim worshippers in New Zealand's worst mass shooting got underway on Monday under tight security.

Australian white supremacist Brenton Tarrant showed little emotion as he appeared before the Christchurch High Court handcuffed and dressed in gray prison clothes.

Tarrant has pleaded guilty to 51 murder charges, 40 charges of attempted murder and one charge of committing a terrorist act. 

Lawyers expect Tarrant to be the first person in New Zealand's history to be imprisoned for life without parole.

The hearing opened with prosecutors describing the attacks in a 26-page summary — the first detailed official account of what happened on that day.

On March 15, 2019, 29-year-old Tarrant shot worshippers at two mosques in Christchurch during Friday prayers, live-streaming the act on social media. He had also uploaded a manifesto shortly before the shootings.

Crown prosecutor Barnaby Hawes said the gunman planned his attack at the city's Al Noor mosque for when the maximum number of people would be present. He had flown a drone over the grounds two months earlier, taking note of the location of entry and exit doors.

The first day of the four-day hearing was also a first opportunity for friends and relatives of the victims to confront the gunman.

Read more: New Zealand rejects US-style armed police squads

"You killed your own humanity, and I don't think the world will forgive you for your horrible crime,'' said Maysoon Salama, the mother of 33-year-old Atta Elayyan who was killed in the attacks.

"You thought you can break us," she added. "You failed miserably.''

High Court judge Cameron Mander is set to hear statements from 66 survivors over the four-day hearing. Tarrant will make his own submission ahead of the verdict.

Sentencing amid media restrictions

The media has been banned from live reporting the proceedings from the courtroom to ensure that any neo-Nazi propaganda from Tarrant, while he is in the dock, does not gain publicity.

Restrictions are also in place on what journalists can report. Any violation of those rules is likely to result in a contempt of court charge.

"The court has a duty, particularly in the context of offending against the Terrorism Suppression Act, to ensure it is not used as a platform... (and) to prevent it being used as a vehicle for further harm," he said.

Read more: Opinion: One year on, New Zealand struggles to make sense of mosque attacks

Prime Minister Jacinda Ardern said it would be a difficult week for many: "I don't think there's anything I can say that is going to ease how traumatic that period is going to be," she told reporters.

There was a heavy police presence outside the courthouse early Monday, with sniffer dogs checking lines of court staff and media lining up at security points. Mental health experts were also on standby. Local media reported that snipers were stationed on the courthouse roof.

The proceedings are being translated live into eight languages to accommodate New Zealand's diverse Muslim community.

dvv/nm (AFP, Reuters)