A New Zealand judge has ordered the accused Christchurch mosque gunman to undergo a mental health assessment to determine if he is fit to face trial for the murder of 50 Muslim worshippers.

Brenton Tarrant is facing 50 murder and 39 attempted murder charges over the 15 March attacks on two mosques.

The 28-year-old Australian will be seen by two health assessors to determine "whether he is fit to stand trial or insane", High Court judge Cameron Mander ruled during a brief hearing in Christchurch.

Tarrant appeared via video link from Auckland, where he is being held in isolation in a maximum security prison following the deadliest massacre in modern New Zealand history.

The suspect - a self-avowed white supremacist - sat motionless throughout the hearing. He was not required to enter a plea.

Several relatives of victims were in the court getting their first glimpse of the man charged with the massacre.

"(I) just want to see what he has to say, what sort of feeling he's got, (his) emotion, to see what his reaction is, good or bad," Yama Nabi, whose 71-year-old father was killed, told Radio New Zealand outside the court.

Tarrant was remanded in custody until his next court appearance on 14 June.

Ahead of the shooting, Tarrant posted a 74-page manifesto on social media in which he identified himself by name and described himself as a white supremacist out to avenge attacks in Europe perpetrated by Muslims.

He live-streamed himself as he opened fire in the packed Al Noor mosque during Friday prayers and then travelled across town to continue the carnage in the suburban Linwood mosque.

Al Noor mosque in Christchurch where most of the victims died in the March 15 attacks

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Temel Atacocugu (C), a survivor of the attacks attended the court hearing

Tarrant was initially charged with one murder count as a holding measure when he made his first court appearance a day after the killings.

However, the charges were updated today to include the names of all 50 who died in the attack and 39 others who were wounded.

Tarrant had sacked a court-appointed lawyer after his first court appearance, raising fears he wanted to represent himself and attempt to use any trial as a propaganda platform.

However, two Auckland lawyers, Shane Tait and Jonathan Hudson, appeared in court on his behalf.

New Zealand Prime Minister Jacinda Ardern called the mosque slaughter a well-planned "terrorist attack" and took immediate steps to tighten the country's gun laws.

The government has also said it will review laws dealing with hate speech and called for social media giants to do more to combat online extremism.

The latest update from health authorities earlier this week said 24 people remained in hospital after the attacks, four of them critical, including a four-year-old girl.