The Australian man accused of murdering 51 people in the terror attacks on two Christchurch mosques last year has admitted he carried out the killings.

Key points: Tarrant will be sentenced at a later date

Tarrant will be sentenced at a later date He carried out the worst terrorist attack ever committed by an Australian

He carried out the worst terrorist attack ever committed by an Australian Jacinda Ardern said she was glad bereaved families had been spared the "ordeal" of a trial

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.

New Zealand Prime Minister Jacinda Ardern met with members of the Muslim community after the mosque attacks. ( Supplied: Christchurch City Council )

New Zealand Prime Minister Jacinda Ardern made a few brief comments following Tarrant's admissions.

"The guilty plea today will provide some relief to the many people whose lives were shattered by what happened on March 15 [2019]," she said.

"These guilty pleas and conviction bring accountability for what happened and also save the families who lost loved ones, those who were injured, and other witnesses, the ordeal of a trial.

"I can't make any further comment given that sentencing is yet to happen."

Tarrant has been in custody since was he arrested on the day of the attack.

He stormed the mosques during Friday prayers, armed 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 Tarrant entering a mosque and opening fire on dozens of people, before fleeing in a vehicle.

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

At the time, authorities urged people not to share or watch the video, and undertook extensive efforts to remove it from the internet.

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.

The court was only notified this week of the defendant's intention to change his plea.

A hearing was hastily arranged despite the curbs in place because of coronavirus.

In a minute, Justice Cameron Mander said he weighed up the public health situation and "considered the need to take the opportunity to progress the matter was particularly acute coming, as it has, at a time when the risk of further delay as a result of COVID-19 was looming as a realistic possibility".

Most victims were not able to attend because of the lockdown.

Janna and Hazim Ezat lost their son Hussein in the Christchurch shootings nearly one year ago. ( ABC News: Mitch Woolnough )

For legal reasons, the nature of the matter could also not be made public in advance.

Families of victims welcome news of changed plea

When someone called Janna Ezat to give her the news, she said she thought she was dreaming.

"I said, 'What?'" she told the ABC.

"And then I jumped from my bed. Yeah, I jumped."

Ms Ezat lost her 35-year-old son Hussein Al-Umari in the attack on the Al-Noor mosque.

She has been struggling to get a hold on her grief, but today she laughed and danced for the first time since the attacks.

"I've been waiting to hear my laugh," she said.

"I missed laughing.

"I just danced to my favourite music with my son's photo in my hand."

Ahmed Jahangir said he also jumped out of bed when he saw the news on a WhatsApp group.

"It was quite surprising and quite shocking for me this morning," he said.

Mr Jahangir was at the Linwood mosque when the gunman struck. He was hit by a bullet in the shoulder and still wears a sling and suffers severe nerve pain from his injury.

Mr Jahangir is a survivor of the shooting. ( ABC News: Mitchell Woolnough )

The father-of-two said initially he felt disappointed to realise there would be no trial in the case, which he said would have been a chance for the accused to "realise what he has done to the community" but he has since come around.

"Being a Muslim we do believe whatever happens it's from Allah," he said.

The victims have been assured that the sentencing hearing will not take place until the coronavirus restrictions are lifted so that they can attend.

Both Ms Ezat and Mr Jahangir hoped to read out their victim impact statements.

Ms Ezat said she had only one message for the killer: "Forgiveness."

But she wants to see him behind bars for the rest of his life.

Hussein Al-Umari's was killed in the attack on the Al Noor mosque. ( ABC News: Mitchell Woolnough )

"For sure, and I might be dancing if the sentence will be life for good," she said.

"I will dance in front of the judge!"

Royal commission investigating attack

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.