Brenton Tarrant, 29, from New South Wales, also admitted the attempted murder of another 40 people, and one charge related to terrorism.
The gun attacks on the Al Noor Mosque and Linwood Islamic Centre during Friday prayers on March 15, 2019 sent shockwaves around the world.
Responding to his plea, Prime Minister Jacinda Ardern said "the guilty plea today will provide some relief to the many people whose lives were shattered by what happened on March 15". Tarrant entered his plea via video link from his prison cell in Auckland instead of in person, as New Zealand begins a four-week nationwide lockdown to combat the coronavirus pandemic.
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As of right now, the lockdown will be enforced for 21 days. " Social distancing is the only option to combat coronavirus". He added that "many families will be destroyed forever" if the country doesn't get a grip on the outbreak, CNN reports .
New Zealand's deadliest mass shooting took place past year on March 15 when Tarrant opened fire on Muslims attending Friday prayers in Christchurch, broadcasting his attack live on Facebook.
Mander said it was unfortunate the lockdown prevented victims and family members from being able to attend the hearing but the imams were helping to represent them. Tarrant had previously denied all charges and was due to stand trial in June. "I wonder if his guilty plea reduces the likelihood of that", he said in a tweet after the guilty pleas.
"Police appreciate this news will come as a surprise to the victims and the public, some of whom may have wished to be present in the courtroom", he said in a statement. The killer will remain in custody until 1 May 2020, at which time a sentencing date will be set or there will be continued detainment.
Police said Tarrant indicated he wanted to be brought before the court on Tuesday afternoon.