Brenton Tarrant, 29, changed his plea from not guilty to guilty on all charges in an appearance by video link in the High Court at Christchurch this morning.
As well as the 51 murder charges, he also pleaded guilty to 40 charges of attempted murder, and a terrorism charge.
Tarrant's trial, which had been set down for June, will now not take place.
The date for his sentencing is yet to be set. New Zealand police say it will only take place when it is possible for all victims who want to attend to do so.
Thursday's small hearing was held at short notice, on Tarrant's request.
The imams of the two mosques attacked were present, but because of coronavirus restrictions, they were among only 17 people allowed in the courtroom.