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.
PM: He wanted to continue the attack
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.