Police have laid further charges against alleged Christchurch gunman Brenton Tarrant this afternoon.

Commissioner of Police 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 15 March 2019 and follows consultation between Police, Crown Law and the Christchurch Crown Solicitors Office," he said.

"An additional murder charge and two additional attempted murder charges have also been filed.

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"Fifty-one charges of murder, 40 of attempted murder and one charge under the Terrorism Suppression Act have now been filed against [the alleged gunman]."

Police met with victim's families and survivors of the March 15 attacks to inform them of the new charges which have been laid.

Bush said they were also updated on the police investigation and the upcoming the court process.

Just over 200 people attended the meeting this afternoon in Christchurch.

"It was led by Detective Superintendent Peter Read and Detective Superintendent Dave Lynch who are joint Senior Investigation Officers, as well as Superintendent John Price, Canterbury District Commander," Bush said.

"Also present were Detective Inspector Greg Murton, officer in charge of the investigation, Detective Senior Sergeant Sarah Illingworth who is managing the family liaison process for Police and a number of Court Victims Advisers."

As the case is before the courts no further commentary will be made by police, Crown Law or the Christchurch Crown Solicitors office on the charges, Bush said.

He also said police are committed to providing the necessary support for the court process which will be "challenging and emotional" for the victim's families and survivors.