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

"A charge of engaging in a Terrorist Act under section 6A of the Terrorism Suppression Act 2002 has now been filed against Brenton Tarrant," police said in a statement.

They said that in addition to the terror charge, the gunman now faces 51 charges of murder and 40 of attempted murder over the 15 March attacks in the South Island city.

New Zealand Prime Minister Jacinda Ardern has characterised the mosque killings a well-planned "terrorist attack" since the day the accused, 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 was made after consultations with prosecutors and government legal experts.

The 28-year-old Australian, is currently in a high security prison undergoing tests to determine if he is mentally fit to stand trial.

His next court appearance is scheduled for 14 June.

Police said they had met survivors and victims' families 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.