Brenton Tarrant, 28, appeared in court on Saturday in relation to the mosque attacks
Australian Brenton Tarrant, 28, was brought to the dock in a white prison shirt and handcuffs. Further charges are expected to be made against him.
PM Jacinda Ardern said Mr Tarrant had a firearms licence and owned five guns, adding: "Our gun laws will change."
Two others are in custody. None of those detained had a criminal record.
Mr Tarrant was described by Australian Prime Minister Scott Morrison as an "extremist, right-wing, violent terrorist".
The suspect, who stood silently during the brief hearing in Christchurch, was remanded in custody without a plea and is due to appear in court again on 5 April.
Ms Ardern called the attack "an act of terror", and officials are still carrying out the identification of the victims.
Ms Ardern said the guns used by the attacker appeared to have been modified, and that the suspect's car was full of weapons, suggesting "his intention to continue with his attack".
Speaking at a news conference on Saturday, she said the suspect had obtained a gun licence in November 2017 that allowed him to buy the weapons used in the attack.
"The mere fact... that this individual had acquired a gun licence and acquired weapons of that range, then obviously I think people will be seeking change, and I'm committing to that."
New Zealand's Attorney-General David Parker said the government would look into banning semi-automatic weapons, but that no final decision had been made. Previous attempts to tighten gun laws in a country with a strong gun lobby and a culture of hunting have failed.
All day on Saturday the people of Christchurch have been turning out to show their rejection of the hate that inspired Friday's horrific attacks.
In ones and twos and in family groups, people have been coming by the hundred to a makeshift memorial set up on the edge of Hagley Park. Outside the two mosques that were attacked, people have been laying more flowers. Many have left hand-written notes. "This is not New Zealand," one read.
At one point a group of young men started quietly singing a traditional Maori song, their heads bowed, eyes closed. The mayor of Christchurch said the killer had come to the city with hate in his heart, to perform an act of terrorism. But she said he did not represent anything about the city.
Still, there are lots of uncomfortable questions for the authorities here. The man now in custody, Brenton Tarrant, made no secret of his support for white supremacy. He had reportedly been planning the attacks for months. And yet he was not on any police watch list. He did not have any trouble getting a gun licence, nor in buying a collection of high-powered weapons.