The unprecedented attacks have seen the country’s threat level raised to high for the first time in its history.
Public events across the nation scheduled for the weekend have been cancelled amid safety fears, with police officers and helicopters on patrol.
“There is no guarantee the risk is limited to Canterbury and we need all New Zealanders to be extra vigilant,” police said.
Former NSW personal trainer Brenton Tarrant, 28, has been charged, initially, with one count of murder.
Tarrant, who was reportedly wearing a tunic and was handcuffed, did not apply for bail or to have his name suppressed and has been remanded in custody without plea until April 5.
Two armed guards escorted Tarrant into the court and he showed no expression as District Court Judge Paul Kellar read one charge of murder to him, the AP reported.
The court appearance lasted only about a minute and he was led back out in handcuffs. He was ordered to return to court again April 5.
After Tarrant left, the judge said that while “there is one charge of murder brought at the moment, it is reasonable to assume that there will be others”.
Two other people arrested are yet to be charged.
“This individual has travelled around the world with sporadic periods of time spent in New Zealand,” Ms Ardern said of Tarrant.
She said inquires were ongoing to establish whether the other two were directly involved.
A fourth person arrested on Friday was a member of the public who was in possession of a firearm, but with the intention of assisting police, she said. They have since been released.
Tarrant identified himself online as behind the shooting that began as hundreds of worshippers gathered for prayers in the South Island city on Friday.
Police have said none of those arrested had appeared on the watchlist of New Zealand or Australian security agencies.
Tarrant’s family approached NSW Police after viewing footage of the terror attack.
Police Commissioner Mick Fuller on Saturday said there were “no active threats” in NSW linked to the attack in New Zealand, however the national terrorism threat level remains at “probable”.