Philip Arps pleaded guilty to two counts of distributing the mosque video and will remain in jail until he is sentenced on June 14.
He faces a maximum penalty of 14 years in prison.
Prosecutors accused the 44-year-old of sending the video to an unknown person and instructing that person to insert crosshairs and to include a kill count.
Prosecutors say he then forwarded the entire chilling 17-minute video to 30 associates.
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New Zealand's chief censor David Shanks banned both the video and a manifesto written by the white supremacist accused of the attack, making it illegal to view, possess or distribute them.