Prime Minister Jacinda Ardern has told families of the victims in the Christchurch mosque attacks that the nation "can never know their grief".

Parliament's Speaker Trevor Mallard has led a multi-faith delegation into the debating chamber this afternoon for a special sitting of Parliament.

The procession came down the Speaker's Corridor and through the Grand Hall before going into the House.

Prayers were offered by each of the religious representatives, beginning with a Muslim prayer.

Statements of condolence, led by Ardern, will be made before the House adjourns for the day.

Ardern told Parliament the 15th of March would now be a date etched in everyone's memories.

Last Friday had now become New Zealand's darkest day.

"They were New Zealanders, they were us. Because they were us, we mourn them," she said.

Ardern said she never anticipated ever having to voice the grief of the nation and had hoped never to.

She spoke directly to the families. "We cannot know your grief but we can walk with you at every stage."

Ardern said the police officers involved in the arrest of the suspect were heroes, but she acknowledged those who lost their lives trying to stop the gunman after he burst in to two mosques and opened fire.

She paid tribute to the first responders who worked and were still working.

"We are proud of your work and incredibly grateful for it."

Ardern said the nation remained on high alert and while there was no specific threat, the country was maintaining its vigilance.

She said there had rightly been questions about how the atrocity had occurred, and anger that it had.

She promised that all those questions would be answered.

"We cannot allow this to happen again."

She said there needed to be a frank examination of the gun laws.

Decisions made at Cabinet yesterday would be announced before the week was out.

One person was at the centre of everything.

"You will never hear mention his name. He is a terrorist.

"Speak the name of those who were lost."

The language of division had existed for decades but the ways they were distributed were new.

It was not good enough to sit back and allow social media platforms to operate. They needed to take responsibility.

National Party leader Simon Bridges said no one could have imagined the terror about to be unleashed.

That 50 worshippers at the two mosques would have their last day of their lives.

"All of us changed forever."

"New Zealand was somewhere where they could find solace in a world full of problems," Bridges said of the immigrants and refugees who died or were injured.

It was not something that happened only to Muslims or Christchurch. It had happened to all New Zealanders, he said.

"Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars," Bridges said, quoting US civil rights leader Martin Luther King.

New Zealand First leader Winston Peters called the perpetrator of the violence a "coward".

An attack on people practising their beliefs was an attack on everyone's beliefs, Peters said.

He said New Zealand was not alone, and he had received many messages of support from Muslim countries.

The gunman's ways "were not our ways".

New Zealand's essential character would not change in the aftermath of the massacre.

Peters praised Ardern's leadership during what was a "massive test of our resolve" and MPs would follow her example.

Green Party co-leader Marama Davidson acknowledged those who had died or were injured.

"You were praying. You were in the most profound state of harmony."

"You were anchored in the collective love of your community."

"Your families have been ripped apart, your hearts broken, your wairua destroyed."

"This is your home, you should have been safe here," she said.

Davidson said the ideology that drove the attacks also wanted to harm other communities.

She told the Muslim community: "We did not protect you. We will do better."

"We need to let you know that we hear you, we get it."

Davidson said it was time to understand that words mattered.