Brenton Tarrant changed his plea from not guilty to guilty in an appearance by video link in the High Court at Christchurch this morning.

The 29-year-old pleaded guilty to 40 charges of attempted murder and a terrorism charge.

Now that Tarrant has admitted the charges, there will be no trial, which had been set down for June.

Sentencing will take place at a date yet to be set, with New Zealand police saying it will take place only when it is possible for all victims who want to attend can do so.

Thursday's small but explosive hearing was held at short notice on the request of Tarrant.

The Imams of the two mosques attacked were present, but because of coronavirus restrictions, they were among only 17 people allowed in the courtroom.

Tarrant has been in custody since was he arrested on March 15, 2019 over the shooting deaths of 51 people.

He stormed the mosques during Friday prayers with several high-powered weapons and live-streamed the attack online, in what is the single-worst terrorist attack carried out by an Australian.

The video showed a gunman entering a mosque and opening fire on dozens of people, before fleeing in a vehicle.

He continued to fire his automatic weapon out the window of the car.

At the time, authorities urged people not to share or watch the video and undertook extensive efforts to remove it from the internet, however, it was initially viewed 4,000 times before being taken off Facebook.

New Zealand's gun laws were changed following the attacks, after it was revealed Tarrant had a gun licence that allowed him to legally obtain weapons.

The country's national security threat level was also raised to high, for the first time.

During earlier hearings, many of which were procedural, family and friends of the victims had packed the courtroom.

In one of his recent appearances, Tarrant had withdrawn his application to move his trial away from Christchurch to Auckland.

His lawyer declined to answer questions on why his client decided to withdraw the application.

A royal commission was also established to examine what government agencies knew about Tarrant in the lead-up to the attacks, what actions were taken, what could have been done to prevent the atrocity and what could be done to prevent future attacks.

As the commission was going to dig deeply into some highly classified, national security information, it was held almost exclusively behind closed doors.