The man accused of shooting dead 51 Muslim worshippers in the Christchurch mosque attacks was formally charged with terrorism for the first time on Tuesday, New Zealand police said.

In addition to the terror charge, Brenton Tarrant also faces 51 charges of murder and 40 of attempted murder over the March 15 attacks that rocked the South Pacific nation.

"The charge will allege that a terrorist act was carried out in Christchurch," police said in a statement.

New Zealand Prime Minister Jacinda Ardern has characterised the mosque killings as "a well-planned terrorist attack" since the day Tarrant, a self-described white supremacist, allegedly carried them out.

But until now the charges against him were less expansive, as New Zealand's Terrorism Suppression Act was only introduced in 2002 and is untested in the courts.

Police said the decision to lay the terror charge more than two months after the attack was made following consultation with prosecutors and government legal experts.

Tarrant, a 28-year-old Australian, is currently in a high-security prison undergoing tests to determine if he is mentally fit to stand trial for the worst massacre in modern New Zealand history.

His next court appearance is scheduled for June 14.

Police said they had met survivors and victims' families on Tuesday to explain the additional charges.

"Police are committed to providing all the support necessary for what will be a challenging and emotional court process to come for the victim’s families and survivors of the attack," they said.

Ahead of the shooting, Tarrant posted a rambling 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.

Ardern's government tightened the country's gun laws in the wake of the attack and has said it will review laws dealing with hate speech.

It has also pushed international efforts to ensure social media giants to do more to combat online extremism, including the so-called "Christchurch Call" unveiled by world leaders and top technology firms in Paris last week.