Brenton Tarrant, who was due to stand trial in June on 51 charges of murder, 40 charges of attempted murder and one count of terrorism, admitted carrying out the attacks in a High Court hearing in Christchurch on Thursday morning.

He will become the first convicted terrorist in New Zealand's history.

Tarrant, who appeared by videolink from Auckland Prison, was remanded in custody until May 1 by presiding judge Justice Cameron Mander.

Arrangements for the court hearing were made on Tuesday after Tarrant indicated, through his counsel, he wished to be brought before the court.

TVNZ, one of just a handful of media outlets allowed into Thursday's hearing due to the New Zealand coronavirus lockdown, reported every name of Tarrant's victims was read to the 29-year-old before he entered his guilty plea.

Justice Mander imposed a short embargo on reporting the news to allow for survivors and the families of victims to be informed, before the wider public.

"It is regrettable that the COVID-19 restrictions that presently apply do not permit victims and their families to travel to be present in the courtroom when the defendant entered his pleas of guilty," he said.

Imams from the Al Noor mosque and Linwood Islamic Centre, the two mosques targeted in the shooting, were both in the court.

A sentencing date is yet to be set, with Police Commissioner Mike Bush saying it was likely to be delayed by the COVID-19 pandemic. Consideration will be given to having as many survivors and families of victims attending the sentencing hearing.

"This is New Zealand's largest ever criminal prosecution," Mr Bush said.

"While the sentencing hearing is still pending, today's guilty pleas are a significant milestone in respect of one of our darkest days."

Prime Minister Jacinda Ardern, upon learning of the development, said her reaction was to "let our a massive sigh of relief".

"I know that there will be a certain sense of relief that the whole nation, and particularly our Muslim community, are being spared from a trial that could otherwise have acted as a platform," she said.

"Nothing will bring their loved ones back but this is a small reprieve."

Tarrant is expected to be sentenced to life imprisonment.

Tarrant, who describes himself as a far-right fascist, developed his radical views during conversations in dark corners on the internet. They were then consolidated on trips to Europe.

He grew up in the northern NSW town of Grafton, moving to New Zealand in 2017 and settling in the South Island town of Dunedin.

Tarrant travelled north to Christchurch on March 15 to carry out his attack, timed to capture worshippers at their Friday prayers.

Tarrant published a manifesto detailing his hateful views before the shooting, and wore camera equipment that allowed him to live-stream his horrific crimes.

Both the live-stream recording and the manifesto are illegal to obtain or read in New Zealand.

He was apprehended by police after fleeing Linwood Islamic Centre.

Christchurch mourned the first anniversary of the attack earlier this month, though a national remembrance service was cancelled due to coronavirus restrictions.

Australian Associated Press