Brenton Harrison Tarrant has today made a shock admission that he was the lone gunman who murdered 51 Muslims at two Christchurch mosques on March 15 last year.

The 29-year-old Australian entered the guilty pleas at a special, hastily-arranged High Court hearing in Christchurch this morning.

Tarrant, who appeared from prison on a screen via audio-visual link (AVL) wearing a grey prison sweatshirt, pleaded guilty to all 51 murder charges.

He also admitted 40 charges of attempted murder relating to the two attacks at Masjid Al Noor and Linwood Islamic Centre on March 15 last year - and pleaded guilty to one charge of engaging in a terrorist act laid under the Terrorism Suppression Act 2002.

New Zealand's worst-ever act of terrorism was filmed by the shooter and livestreamed on Facebook, leading to gun reforms and a global political summit initiated by New Zealand Prime Minister Jacinda Ardern to eliminate terrorist and violent extremist content online.

Justice Cameron Mander convicted Tarrant on all charges today and remanded him in custody to a nominal date of May 1 when it's expected that a sentencing date will be set - once coronavirus-imposed court restrictions are eased.

None of the victims knew about today's remarkable, hurriedly-organised hearing.

The city's two imams, Imam Gamal Fouda of Masjid Al Noor and Imam Alabi Lateef from Linwood Islamic Centre, were asked to come to court today to witness proceedings on behalf of their Muslim communities.

But it's understood that even they didn't know what it was going to be about.

Fouda wept while the court registrar took several minutes to read aloud all 51 murder victims named on the Crown charge list, before asking Tarrant if he pleaded guilty or not guilty.

After Tarrant questioned one of the murder victim's names and it was clarified by the judge, he replied: "Oh OK, yes guilty."

Tarrant listened intently while the names were read out. He betrayed no emotion.

The 40 attempted murder charges, which again included the reading aloud of all of the victims, were also put to Tarrant. When he was asked his plea, he said: "Guilty."

He also pleaded guilty to one charge of engaging in a terrorist act on March 15 last year.

The Crown's summary of facts, which outlines the offending, will be read out at sentencing.

Justice Mander called for a pre-sentence report and victim impact statements.

At the conclusion of the short hearing, Tarrant did not say anything as the AVL link was ended.

The June trial date has been vacated.

How today's hearing came about

Justice Mander then took time to explain to those inside court just how today's hearing came into being.

Earlier this week, the courts – which are deemed an essential service during the lockdown - received an indication from Tarrant's Auckland-based defence lawyers Shane Tait and Jonathan Hudson that he may seek to change his plea to the charges.

Yesterday, the lawyers received formal written instructions from Tarrant confirming he wanted to change his pleas and court staff started making urgent preparations for the case to be called as soon as possible – while doing so amidst the ramping up of the coronavirus lockdown which came into effect at midnight.

While admitting it was "regrettable" to hold the hearing without any victims or family members being present, Justice Mander felt it could be managed if numbers inside the courtroom were severely limited. He said today signalled a "very significant step" in bringing finality to the proceedings.

In the large Christchurch Justice Precinct courtroom this morning, there were only 17 people.

Five journalists from New Zealand's major media organisations were granted permission to be in court today. They were spread out across the courtroom as part of coronavirus personal distancing guidelines.

The two imams were also there, with a third member of the Muslim community, Detective Inspector Greg Murton, in charge of the investigation, Christchurch Crown Solicitor Mark Zarifeh and Crown prosecutor Barnaby Hawes, a court registrar, court taker, Ministry of Justice media representative, and court security officer.

The contents of today's hearing, which concluded at 10.30am, were subject to a court-imposed one-hour embargo to allow court and police victim support advisors to tell shooting survivors and family members about today's shock guilty pleas.

Justice for victims 377 days after attacks

Tarrant was arrested minutes after leaving the Linwood mosque by two police officers scrambled into action from a training day.

He appeared in court the following morning charged with murder.

At this third court appearance at the High Court on June 14, he was deemed fit to stand trial and entered not guilty pleas to all charges.

A jury trial was scheduled to begin at the High Court in Christchurch on June 2 this year.

The trial loomed as one of the largest in New Zealand's criminal justice history.

It was expected to last up to six weeks and, at one point, was thought to call as many as 300 witnesses. Much of the evidence would have been extremely confronting and distressing for the jury members – not to mention the survivors and family members of shooting victims.

TARRANT'S TIMELINE OF APPEARANCES

Saturday March 16 2019 - Christchurch District Court

Tarrant makes his first appearance before a judge. His name is allowed to be published but his image is suppressed for now. He is charged with one count of murder but police say it is inevitable more charges will follow.

April 5 2019 - Christchurch High Court

More charges are laid against Tarrant. As of this date he faces 51 murder and 39 attempted murder charges. Justice Cameron Mander ordered two mental health assessments to establish whether Tarrant is fit to enter a plea and stand trial.

14 June 2019 - Christchurch High Court

Tarrant enters not guilty pleas to 51 charges of murder, 40 of attempted murder and a charge under the Terrorism Suppression Act 2002. A trial date is set for May 2020. At this hearing Justice Mander rules Tarrant is fit to stand trial. "No issue arises regarding the defendant's fitness to plead, to instruct counsel, and to stand his trial. A fitness hearing is not required," he said.

3 October 2019 - Christchurch High Court

Tarrant appears via audio visual link. The hearing is supposed to be around an application to move his trial out of Christchurch. However once the relevant parties are assembled in court Justice Mander confirms the application has been withdrawn.

12 September 2019

Justice Cameron Mander issues a minute confirming that Tarrant's trial date will be changed from May 2020 to June. The minute came after Crown prosecutor Mark Zarifeh filed a memorandum advising that "difficulties have arisen with the trial date because it clashes with the Islamic holy month of Ramadan". Ramadan occurs over the month of May next year, affecting many of the trial's witnesses.

10 December 2019 - Christchurch High Court

Pre-trial callover held. Tarrant's appearance is excused.

24 February 2020 - Christchurch High Court

Tarrant appears via audio visual link for a pre-trial hearing. The nature and content of that hearing is suppressed.

26 March 2020 - Christchurch High Court

Tarrant pleads guilty to all charges.