Authorities updated the death toll to 49 and said more than 20 were seriously wounded in what they were calling a carefully planned racist attack.
Three men and one woman now in police custody over the shootings at two mosques in Christchurch.
Authorities reported defusing improvised explosive devices found on vehicles after the shootings.
Prime Minister Jacinda Ardern called the events Friday "one of New Zealand's darkest days."
WELLINGTON, New Zealand — Prime Minister Jacinda Ardern says 40 people have been killed in an attack at mosques in the New Zealand city of Christchurch.
Four people have been detained, and one is Australian.
Ardern said more than 20 people were seriously injured during the shootings at two mosques during Friday prayers.
Thirty fatalities occurred at the Masjid Al Noor mosque in central Christchurch. Seven of the dead were inside the suburban Linwood Masjid Mosque and three died outside the same mosque.
A man who claimed responsibility for the shootings said in a manifesto that he was a 28-year-old white Australian who came to New Zealand only to plan and train for the attack.
Witness Len Peneha said he saw a man dressed in black enter the Masjid Al Noor mosque in central Christchurch at about 1:45 p.m. and then heard dozens of shots, followed by people running from the mosque in terror.
Peneha, who lives next door to the mosque, said the gunman ran out of the mosque, dropped what appeared to be a semi-automatic weapon in Peneha's driveway, and fled.
Peneha said he then went into the mosque to try and help.
"I saw dead people everywhere. There were three in the hallway, at the door leading into the mosque, and people inside the mosque," he said. "It's unbelievable nutty. I don't understand how anyone could do this to these people, to anyone. It's ridiculous."
"I've lived next door to this mosque for about five years and the people are great, they're very friendly," he said. "I just don't understand it."
He said the gunman was white and was wearing a helmet with some kind of device on top, giving him a military-type appearance.
Ardern held a press conference into the shooting at two mosques in Christchurch, describing it as "one of New Zealand's darkest days."
Australian Prime Minister Scott Morrison has confirmed that one of the four people taken into custody in New Zealand's mosque shootings is an Australian. A man who claimed responsibility for the shootings said in a manifesto that he was a 28-year-old white Australian who came to New Zealand only to plan and train for the attack.
New Zealand police said they had arrested four people. Morrison on Friday confirmed one of those who were arrested was an Australian-born citizen. He described the gunman as "an extremist, right-wing, violent terrorist."
Ardern said: "For now my thoughts and I'm sure the thoughts of all New Zealanders are with those who have been affected their families."
Parts of Christchurch were put on lockdown as armed police were deployed after receiving reports of shots fired in the city center at 1:40 p.m. Friday local time.
Speaking to CNN, witness Mohan Ibn Ibrahim said he was inside the mosque when the shooting began and that he heard the gunman "continuously shooting for ten to 15 minutes."
"I was in the mosque. It's a big mosque and there were more than 200 people inside. The gunmen came from the backside. Gunshots went on for a long time. We had to jump the wall to escape. I saw lots of broken glass and bricks on the backside of the mosque," he said.
"I came to the street I saw one person got shot on his chest," he said, adding that the ambulance and police then arrived on the scene.
He said that he had a friend in another mosque in the area had told him a gunman had opened fire there as well and five people were dead.
"I could not contact two of my friends who are in the mosque as well," he said.
A spokesperson at Christchurch Hospital told CNN that "multiple" casualties had been sent there, but did not confirm the number.
Police are aware of a video shared online and not verified by CNN that purports to show a gunman walking into an unnamed mosque and opening fire.
Witness Mohammed Nazir told TVNZ that he "heard the big sound, the gun" at about 1.45 p.m.
"The second shot, I run, lots of people were sitting on the floor," he said. "The gun was (at the) front door, I run behind the mosque and I was sitting behind... and I called the police."
According to TVNZ, Nazir saw multiple deaths and injuries, including three women on the ground outside the mosque.
Bush said in a statement that "all Christchurch schools have been placed into lockdown."
A video posted on Twitter showed Bangladesh's cricket team walking away from the mosque. Speaking to CNN, the team's coach said their bus had pulled up at the mosque when shooting happened.
Christchurch is a coastal city of 404,500 residents. It is the second most-populous city in New Zealand behind Auckland and Wellington. It has an agricultural economy.
In 2010 and 2011 the city suffered a series of devastating earthquakes, with the most destructive at 6.3 magnitude, which killed nearly 200 people and destroyed thousands of buildings.