One man was arrested and charged with murder in what appeared to be a carefully planned racist attack. Police also defused explosive devices in a car.
Two other armed suspects were being held in custody. Police said they were trying to determine how they might be involved.
"It is clear that this can now only be described as a terrorist attack," Ardern said.
Police took three men and a woman into custody after the shootings, which shocked people across the nation of 5 million people. Police later said one of the arrests didn't relate to the shootings.
Authorities have not specified who they detained, but said none had been on any watch list. A man who claimed responsibility for the shootings left a 74-page anti-immigrant manifesto in which he explained who he was and his reasoning for the attack. He said he was a 28-year-old white Australian and a racist.
Australian Prime Minister Scott Morrison confirmed that one of the people detained was an Australian-born citizen.
Bush said police had found two improvised explosive devices in one car, a clarification from an earlier statement that there were devices in multiple vehicles. He said they had disabled one and were in the process of disabling the second.
The deadliest attack occurred at the Al Noor mosque in central Christchurch at about 1:45 p.m., when 41 people were killed.
Witness Len Peneha said he saw a man dressed in black enter the mosque and then heard dozens of shots, followed by people running from the mosque in terror.
He said the gunman was white and was wearing a helmet with some kind of device on top, giving him a military-type appearance.
A video that was apparently livestreamed by the shooter shows the attack in horrifying detail. The gunman spends more than two minutes inside the mosque spraying terrified worshippers with bullets again and again, sometimes re-firing at people he has already cut down.
He then walks outside to the street, where he shoots at people on the sidewalk. Children's screams can be heard in the distance as he returns to his car to get another rifle.
The gunman then walks back into the mosque, where there are at least two dozen people lying on the ground. After walking back outside and shooting a woman there, he gets back in his car, where the song "Fire" by English rock band "The Crazy World of Arthur Brown" can be heard blasting from the speakers. The singer bellows, "I am the god of hellfire!" and the gunman drives away. The video then cuts out.
During a second shooting at the Linwood mosque about 3 miles from the Al Noor mosque, seven people were killed.
The man who claimed responsibility for the shooting said he came to New Zealand only to plan and train for the attack. He said he was not a member of any organization, but had donated to and interacted with many nationalist groups, though he acted alone and no group ordered the attack.
He said the mosques in Christchurch and Linwood would be the targets, as would a third mosque in the town of Ashburton if he could make it there.
He said he chose New Zealand because of its location, to show that even the most remote parts of the world were not free of "mass immigration."