In what Prime Minister Jacinda Ardern has called "one of New Zealand's darkest days", the gunman armed himself with a semi-automatic rifle and stormed into two mosques on 15 March last year where he opened fire on worshippers at Friday prayer.
He was arrested soon after and initially pleaded not guilty to 51 murders, 40 attempted murders, and committing a terrorist act.
In March this year he changed his plea to guilty.
Prominent Christchurch lawyer Nigel Hampton said the "terrifying" crime probably "needs an extraordinary sentence", while law professor Chris Gallavin said "he'll never see the light of day again".
Judge Mander has said the sentencing process was important for the victims, and "finality and closure is considered by some as the best means of bringing relief to the Muslim community".
But he has also imposed extraordinary reporting restrictions on the media to ensure that if the 29-year-old gunman spouts neo-Nazi propaganda from the dock it does not gain publicity.
Difficult week for many
Judge Mander will inform media after each court session what can and cannot be reported, with any breaches 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.
Ms 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 will be increased police around the courthouse and victim support workers will be present, with local mental health specialists on standby for referrals.
It is part of a massive logistical exercise that includes live translation of proceedings into eight languages to accommodate New Zealand's diverse Muslim community.
The attack led to a crackdown on firearm ownership in New Zealand and Ms Ardern initiated a global campaign aimed at eliminating terrorist and violent extremist content online.
Possession of a manifesto posted online by the gunman shortly before the attacks, and video recordings of the shootings are banned in New Zealand.
The longest sentence handed down in New Zealand, which no longer has the death penalty, is a life sentence with a minimum non-parole period of 30 years for a man who murdered three people in 2001.
For people in Australia, mental health support is available at Beyond Blue.org.au. Embrace Multicultural Mental Health supports people from culturally and linguistically diverse backgrounds.