Brenton Tarrant killed 51 people and wounded dozens more at two mosques in Christchurch during Friday prayers on 15 March 2019.
The 29-year-old white supremacist, who live-streamed part of his attack, had originally pleaded not guilty and was due to stand trial on 2 June this year.
He was charged with 51 counts of murder, 40 counts of attempted murder and one count of terrorism.
The attack was New Zealand's deadliest in modern history and forced the government to bring in new legislation banning most semi-automatic weapons.
A sentencing date is yet to be set but Tarrant is facing life imprisonment over the charges.
The plea change came during a hastily arranged court hearing with the country beginning a four-week lockdown over the coronavirus outbreak.
Tarrant appeared in court from his jail cell through a videolink and only a few people were allowed inside the courtroom.