The country's prime minister Jacinda Ardern confirmed the total and said at least 20 others had been seriously injured.
New Zealand Police had earlier said that four people - three men and one woman - were in custody in relation to the attacks.
Australia's prime minister Scott Morrison said one of those arrested was an Australian citizen.
M6 closed in both directions after vehicle transporting sheep overturns following crash between Preston and Lancaster
Lytham St Annes care home boss appears in court after death of residents
The Old Vicarage Care Home: special measures status for Lancashire care home where resident was left lying on floor after falling out of bed
Morecambe 'neighbour from hell' back in court after breaching order
Preston Brick Veil Mosque inquiry closes as debate rages over whether the place of worship should be permitted
A number of improvised explosive devices found on vehicles after the shootings were defused by police.
Ms Ardern said the offender is in custody, adding: "So I can give that assurance, he has been apprehended. He is also accompanied by two other associates."
When asked about the attackers not being on intelligence agency watchlists, she said it was an indication they "had not acted in a way that warranted it".
Asked about the ages of victims, she said: "I will have been amongst other members of the public who will have seen the footage as the injured were being brought to Christchurch A&E and you certainly can see from that footage there is a real range of ages there.
"I imagine that these would have represented particular brothers, fathers, sons."
She added: "We have undoubtedly experienced an attack today that is unprecedented, unlike anything that we have experienced before.
"But, as I say, New Zealand has been chosen because we are not a place where violent extremism exists.
"We reject those notions and we must continue to reject them. This is not an enclave for that kind of behaviour, for that kind of ideology.
"We will and must reject it. This is a place where people should feel secure and will feel secure.
"I am not going to let this change New Zealand's profile, none of us should."
Officers responded to reports of shots fired in central Christchurch at about 1.40pm local time (12.40am GMT), and urged people in the area to stay indoors.
All schools in the city were put into lockdown as the situation unfolded.
Mosques in Deans Avenue and Linwood Avenue were targeted in the attack, but police urged all mosques across New Zealand to shut their doors in the wake of the incident.
Ms Ardern had earlier called it "one of New Zealand's darkest days".
"What has happened here is an extraordinary and unprecedented act of violence," she said.
Witness Mohan Ibrahim said he was one of 200 people in the Masjid Al Noor mosque on Deans Avenue when he heard shots fired.
He told the New Zealand Herald: "At first we thought it was an electric shock but then all these people started running."
"I still have friends inside," he added.
"I have been calling my friends but there are many I haven't heard from. I am scared for my friends' lives."
Members of the Bangladesh cricket team, currently on tour in New Zealand, said on social media that they had nearly been caught up in the tragedy.
Tamim Iqbal tweeted: "Entire team got saved from active shooters!!! Frightening experience and please keep us in your prayers", while Mushfiqur Rahim said: "Alhamdulillah Allah save us today while shooting in Christchurch in the mosque...we r extremely lucky...never want to see this things happen again.... pray for us".
New Zealand Police urged people not to share "extremely distressing footage" relating to the incident that was circulating online.
"It's very disturbing, it shouldn't be in the public domain," a spokesman said.
A spokesman for the Foreign Office said the British High Commission in Wellington was in contact with the New Zealand authorities about the incident.
"British nationals in the area are advised to remain vigilant and follow the advice of local authorities," he said.
"The British High Commission in Wellington is in contact with the New Zealand authorities and urgently seeking further information."