The man who killed 51 people at two mosques in New Zealand in 2019 meticulously planned his shooting rampage to maximise casualties, a prosecutor said at the start of sentencing hearings on Monday.

Australian Brenton Tarrant, 29, has pleaded guilty to 51 murder charges, 40 charges of attempted murder and one charge of committing a terrorist act in relation to the massacre in the city of Christchurch which he livestreamed on Facebook.

He faces a life prison term, possibly without parole in what would be a first for New Zealand, when a High Court judge sentences him later this week.

Handcuffed and dressed in grey prison clothes, Tarrant sat with hands clasped for most of the morning's proceedings. He showed little emotion, and looked directly at those delivering victim impact statements.

Crown prosecutor Barnaby Hawes said Tarrant told police after his arrest that he wanted to create fear among the Muslim population.

"He intended to instil fear into those he described as invaders, including the Muslim population or more generally non-European immigrants," Hawes said.

Tarrant also expressed regret for not taking more lives and had planned to burn the mosques down, Hawes said.

The shooter spent years purchasing high-powered firearms, researched mosque layouts by flying a drone over his primary target, and timed his March 15 attacks to maximise casualties, the prosecutor said.

The shooter went to Christchurch about two months before the attack and flew a drone directly over the Al Noor mosque, with a focus on its entry and exit points.

While most of Tarrant's victims were at Al Noor mosque, he attacked a second mosque before being detained en route to a third.

The attacks prompted a global outpouring of grief as well as scrutiny, with regulations imposed on online platforms after the then 28-year-old live-streamed the mosque shootings shortly after uploading a manifesto.

Gamal Fouda, imam of Al Noor mosque, told Tarrant that he was "misguided and misled" and that his hatred was unnecessary.

"I can say to the family of the terrorist that they have lost a son and we have lost many from our community too," Fouda said.

"I respect them because they are suffering as we are."

Security is tight outside the court, with police dogs pacing the streets and snipers on rooftops, television footage showed.

With social distancing measures in place due to the re-emergence of coronavirus in New Zealand, a small public gallery is at the main court, with survivors and families of victims viewing the proceedings in seven overflow court rooms.

Live reporting from the courtroom is banned, and other restrictions have been put in place on what the media can report.

High Court judge Cameron Mander said that he had received more than 200 victim impact statements, along with submissions from various organisations.

"I have read them all," said Mander, who added that he would not sentence Tarrant before Thursday morning after survivors and family members of victims had an opportunity to address the court.

A murder conviction carries a mandatory sentence of life in prison. The judge can impose a life term without parole, a sentence that has never been used in New Zealand.