New Zealand police filed terrorism charges against Christchurch gunman Brenton Tarrant on Tuesday.

New Zealand Police Commissioner Mike Bush said a charge of engaging in a terrorist act under section 6A of the Terrorism Suppression Act 2002 has been filed.

The charge will allege that a terrorist act was carried out in Christchurch on March 15. The decision came after consultation between the police, the Crown Law office and the Christchurch Crown Solicitors office, the police said.

An additional murder charge and two additional attempted murder charges have also been filed. Fifty-one charges of murder, 40 of attempted murder and one charge under the Terrorism Suppression Act have now been filed against Tarrant, Bush confirmed.

New Zealand police on Tuesday afternoon met with survivors and victims' families of the Christchurch attack to inform them of the new charges filed against the gunman and update them on the ongoing investigation and the court process to come.

About 200 people attended the meeting in Christchurch.

"New Zealand police are committed to providing all the support necessary for what will be a challenging and emotional court process to come for the victims' families and survivors of the attack," said Bush.

On March 15, the alleged gunman attacked two Christchurch mosques and killed 51 people.