But experts warn it could turn his trial into a soapbox and complicate the case.

Brenton Tarrant, 28, will face an unprecedented count of engaging in a terrorist act when he appears again in court on June 14, police announced on Tuesday.

They've also laid three other new charges over the March 15 shootings at two mosques, meaning Tarrant will face 51 counts of murder and 40 of attempted murder as well.

Police met with about 200 people from families of victims and survivors this week to announce the charges and discuss progress in the case.

The prosecution is now expected to act as the first test case of New Zealand's terrorism laws, which were introduced in 2002 but have in past proved unwieldy.

University of Auckland law professor Bill Hodge said the charge was significantly more complex than murder, had never been tested in a prosecution in New Zealand and the law appeared to be more about overseas terrorism.

"This is new territory. I'm not sure that parliament intended it for this kind of thing," he told AAP.

"I think it feels good and I sympathise with the motives of the prosecution ... but in terms of getting a clean prosecution, it suddenly gets messy."

The requirements of the charge would also mean Tarrant, a suspected white supremacist, could potentially use his defence at trial as a platform or to read out his "manifesto" - which could be ruled irrelevant in a murder case, Dr Hodge said.

"It will give him a chance to use the trial as a platform because he can say: 'I'm not a terrorist, I'm a hero''."

But Mohamed Hussein Mostafa, whose father was killed at Al Noor Mosque, said he was glad the attack was being treated a terrorist act, given that the Muslim community had often been vilified as possible perpetrators of violence since the 9/11 attacks.

"It will cement in people's minds that terrorism has no race or religion," he told Reuters.

"I'm happy that he will be made an example of ... so that such an atrocity may never happen again on our shores."

Tarrant is being held in New Zealand's only maximum-security jail, in Auckland, and appeared in Christchurch's High Court by video at his last hearing in April.

He was ordered to undergo mental health assessment before his next appearance.

Prison authorities have confirmed he has had no access to television, radio, newspapers or visitors.

The toll from the attack rose to 51 earlier this month with the death of a Turkish man in Christchurch hospital, nearly 50 days after the shooting.

Australian Associated Press