New Zealand police have laid terrorism charges against the Australian man arrested over the Christchurch mosque attacks.

Brenton Tarrant, 28, will face an unprecedented count of engaging in a terrorist act when he returns to court on June 14, police announced on Tuesday afternoon.

Police have also laid three other additional charges over the March 15 shootings at two mosques, meaning Tarrant will face 51 counts of murder and 40 of attempted murder in total.

Victims’ families and survivors of the attacks were told about the new charges at a meeting with police on Tuesday afternoon.

The move means the former NSW resident is the first person charged under New Zealand’s anti-terror laws.

“A charge of engaging in a Terrorist Act under section 6A of the Terrorism Suppression Act 2002 has now been filed against Brenton Tarrant,” New Zealand police said in a statement on Tuesday.

“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.”

The country’s terrorism legislation has in the past proved unwieldy and complex. Legal experts have also previously warned terror charges might complicate the prosecution for no material difference in outcome if Tarrant is convicted.

He stormed the mosques with high-powered weapons and live-streamed the attack online.

Suspected white supremacist Tarrant is being held in New Zealand’s only maximum-security jail, in Auckland. He appeared in Christchurch’s High Court by video at his last hearing in April.

He was ordered to undergo mental health assessment before his next appearance.

Prison authorities have confirmed he has had no access to television, radio, newspapers or visitors.

The March 15 attacks in Christchurch were New Zealand’s worst postwar mass shootings. Previously the worst was in 1990, when a gunman killed 13 people in a rampage after a dispute with a neighbour in a small town near Dunedin on the South Island.

-with AAP