The identities of the other victims have not yet been released.
A total of 48 people were wounded in the shootings. Among those injured are two young boys - aged two and 13. Eleven of those being treated at Christchurch Hospital are in a critical condition in intensive care, chief of surgery Greg Robertson said.
Bangladesh, India and Indonesia all say some of their citizens were killed in the shooting and others are unaccounted for.
On Saturday, Christchurch Mayor Lianne Dalziel expressed "revulsion" at this "act of terrorism" and voiced solidarity with the victims.
"We have welcomed new people into our city. They are our friends, they are our neighbours," she told reporters. "We want to come together to support them."
Police say they recovered firearms from both mosques, and explosive devices were found in a car belonging to one of the suspects.
According to the latest census figures, Muslims make up about 1.1% of New Zealand's population of 4.25 million.
Numbers rose sharply as New Zealand took in refugees from various war-torn countries since the 1990s.
The main suspect
The main suspect had "travelled around the world with sporadic periods of time spent in New Zealand", Prime Minister Ardern told reporters.
"I would not describe him as a long-term resident," she said, without formally identifying him.
"The offender was in possession of a gun licence. I'm advised that this was acquired in November of 2017," Ms Ardern said.
She said New Zealand intelligence services had been stepping up investigations into far-right extremists, but added: "The individual charged with murder had not come to the attention of the intelligence community nor the police for extremism."
In the wake of Saturday's attacks, social media accounts in the name of Brenton Tarrant were used to post a lengthy, racist document in which the author identified the mosques that were later attacked.