NZ Police Commissioner Mike Bush said 41 people had died at the Al Noor Mosque in central Christchurch and seven at Linwood Mosque. One other victim died after being taken to Christchurch Hospital.

New Zealand Prime Minister Jacinda Ardern said the massacre could only be described as a terrorist attack. She said it was "one of New Zealand's darkest days".

Mr Bush said four people had been arrested, three men and a woman. One of the men, aged in his twenties, was charged with murder on Friday night.

He is due to face court on Saturday.

Following the attack, a number of improvised explosive devices were attached to vehicles and disarmed.

"The attackers were apprehended by local police staff, there have been some absolute acts of bravery," Mr Bush said.

Police were not aware of other people being involved but could not assume that there were not others at large, he said.

One of the shooters was an Australian man from Grafton, who called himself 'Brenton Tarrant' on Twitter and who live-streamed disturbing footage of him carrying out the attack. He was one of the four people arrested and in custody.

Ms Arden said the attack appeared to have been well planned.

"These are people who I would describe as having extremist views, that have absolutely no place in New Zealand, and in fact have no place in the world," she said.

"While we do not have any reason to believe at this stage that there are any other suspects, we are not assuming that at this stage. The joint intelligence group has been deployed and police are putting all of their resources into the situation. The Defence Force are currently transporting additional police staff to the region."

She said many of those in the mosque were likely to be migrants - but they were "us" - New Zealanders.

"There is no place in New Zealand for extreme violence."

"We are potentially still dealing with an evolving situation. And again, as I say, across multiple sites."

Australian Prime Minister Scott Morrison said: "We are outraged and we stand here and condemn absolutely the attack that occurred today by an extremist, right-wing violent terrorist who has taken the lives, stolen lives, in a vicious, murderous attack that has claimed so many New Zealanders."

Mr Morrison said he had no information to suggest any Australians had been killed but noted it was "very early on" and the situation was unfolding.

A witness, with blood splatters across his shirt, said the shooter changed magazines seven times after opening fire as the crowd gathered to pray.

''When the shooting started people started rushing out, and the door was closed, and the guys came on them and started shooting them,'' he said, describing how he hid under a bench and pretended to stop breathing.

''He went to all the different [rooms] and he shot everyone.''

Several hundred people were inside and witnesses reported seeing ''blood everywhere''.

Others described seeing as many as 40 people injured.

''I heard a big sound of the gun. And a second one, I ran. Lots of people were sitting on the floor,'' one told TVNZ.

''The floor. There's lots of blood on the floor, you can see when you go in.''

Meanwhile, Australia's under-19s women's cricketers will return home from Christchurch with their tour of New Zealand cancelled after the shootings.

Australia's youngsters were due to play a one-dayer in Lincoln on Saturday followed by a Twenty20 at the same Christchurch ground on Sunday.

Cricket Australia have been in constant contact with the team.

The gunman identified himself as "Brenton Tarrant" - a white, 28-year-old Australian-born man, who livestreamed for 16 minutes as he shot victims in the mosque - and wrote a 37-page manifesto declaring his intentions.

In the video, he says: "Let's get this party started," as he starts his car and travels to the mosque.

He is seen wearing army-like gear and uses GPS on his phone to direct him there.

As he travels along the driveway near to the mosque, music is playing in his car.

He then parks, gets a gun out of his car and walks towards the mosque.

He opens fire several times before throwing the gun to the ground.

He then picks up another gun and continues to fire, shooting one person as they attempt to crawl away and fires at another person as they run away.

He then gets into another room where he fires at people trying to hide from him. He continues to fire at the people as they lie on the ground.

He then exits the mosque running back to his car firing at people on the street before taking another gun out of the boot of his car and running back to the mosque and firing through the front entrance.

He then re-enters the mosque, again shooting at people lying on the ground from close range as he walks around a room.

Once outside, the man shoots again down the street.

One woman can be seen lying on the road yelling. "Help me, help me."

The man then puts another address into his car's GPS and shoots at several people while in his car.

The gunman's rifles had "Alexandre Bissonette" and "Luca Traini" written on them - the names of mosque shooters in Italy and Canada.

Christchurch Mayor Lianne Dalziel said she was "shocked beyond words" by the shootings".

"I would never have expected anything like this to happen in Christchurch. I'd never expect this to happen in New Zealand," she said.

"I am getting messages from around the country, and from around the world. I know that everyone is shocked."

Shots were reported to have been fired at Christchurch Hospital, which activated its emergency plan. Multiple patients were taken to the hospital and local medical facilities.

Patient injuries ranged from minor through to those with critical gunshot wounds.

A number of helicopter and road ambulances were on standby to provide support, including transporting patients to other ICUs around the country.

About 20 armed police cleared buildings in Linwood, across from the Eastgate Mall.

Police warned people to stay indoors and to report any suspicious behaviour to 111.

The Civil Aviation Authority declared Christchurch City a controlled air space.

All Christchurch schools were put on lockdown, including Christchurch Girls' and Christchurch Boys' high schools and Hagley Community College.

Parents of Christchurch Girls' students were sent text messages about 2.05pm, which said: "This is not an exercise."

A man, who would not give his name, said he was praying in the Al Noor Mosque when he heard the shooting start. He escaped, but outside saw his wife lying dead on the footpath.

"My wife is dead," he said, wailing. He was supported by other Muslim men who prayed for him.

Another man said he saw children being shot.

"There were bodies all over me."

A resident on Freyberg Street said she heard "hundreds" of shots and it was "terrifying".

The former president of the Muslim Association of Canterbury, Mohammed Jama, said a man with a gun went into the Al Noor Mosque mosque about 1.40pm on Friday.

He saw about four people injured and two lying on the ground. He did not know if they were alive or dead.

A photographer said he could see three seriously injured people on the doorstep of the mosque.

'The players just started running when they heard the shots': cricketers flee mosque

"A serious and evolving situation is occurring in Christchurch with an active shooter," the police commissioner said as the events were unfolding.

"Police are responding with their full capability to manage the situation, but the risk environment remains extremely high.

"Police recommend that residents across Christchurch remain off the streets and indoors until further notice. Christchurch schools will be locked down until further notice."

Police cleared Cathedral Square, where thousands of children were rallying for action on climate change. Armed police were also trying to clear Deans Avenue and Hagley Park.

A reporter at the scene said the mood was very tense.

A nearby business owner said a lot of of people were praying at the mosque at the time.

"I heard 20 or more gunshots. I thought something happened and the people were running on the other side of Deans Avenue and now I can see so many police," he said.

A man who ran from the mosque told him the shooting happened there.

One woman in the area said she was told by police there was a shooter in Hagley Park.

Another witness reported hearing at least 20 gunshots and saw what she thought were three people on the ground.

A witness said he saw two people walking out of the Al Noor Mosque. A car was chasing the people along Deans Avenue and the people in the car started shooting at them, he said.

A person at the scene said armed police were searching for someone in North Hagley Park. Two ambulances had arrived.

Another witness said she heard a "bunch of shots" from a building on Deans Avenue. She saw police on the corner of Lester Lane and Deans Avenue. She and others went outside, but police told them to stay inside.

"We heard police telling a gunman to put their hands up," she said.

Syed Ahmed said a man wearing a black motorcycle helmet and camouflage clothing, carrying what he thought was a black assault rifle with white writing, started "indiscriminate shooting" from outside the Linwood Mosque.

The man first shot several elderly people sitting on chairs during prayer, he said.

"They were basically all shot."

He thought at least eight people had been shot, including at least three women and two of his friends. One of those killed had been shot in the head and remained inside the mosque, he said.

The man came through the front door "shouting something" but it "could not be heard over the screaming".

Ahmed said he kept "low on the ground" and crawled to the storeroom before getting away.

Sabir Hussain, who was inside the Linwood Mosque, said he was "doing a prayer when our imam said our brothers have been shot".

Hussain went into the washroom and, through the window, saw one man dead outside.

He knew he "was not safe", jumped out of the window and hid behind a fence until police arrived.

A reporter at the scene said people at the cordon were hugging as they passed through the tape.

One man wearing a prayer robe was covered in blood but appeared to be uninjured, she said.

Farhaan Farheez was praying with about 100 others in the Linwood Mosque when the shooting started.

"I didn't know what a gun sounded like. It is customary when we are praying not to pay attention to the outside world ... Gunshots kept happening and people kept praying.

"I saw two females and four or five males dead and the rest were severe casualties ... The whole mosque was filled with blood and dead bodies. It was like a battlefield."

The 27-year-old moved to New Zealand from Fiji in 2015.

"I am thinking, I have worked so hard to be in New Zealand but what for - to be shot at?"

The Bangladesh cricket team escaped the Al Noor Mosque shooting.

Cricinfo's Bangladesh correspondent Mohammad Isam posted a video to Twitter of the team rushing through Hagley Park away from the mosque.

"Just escaped active shooters!!! Heartbeats pumping badly and panic everywhere!! #ChristchurchMosque," he wrote.

Mario Villavarayen, the team's strength and conditioning coach, said the players were shaken, but fine.

"I spoke to one of them shortly after [the attack]. They didn't see anything but heard gunshots. They were at the ground [Hagley Oval] and just started running.

"The coaching staff were all at the hotel. The players just started running when they heard the shots."

Bangladesh were due to play New Zealand at Hagley Oval on Saturday, but the Test has been called off.