Brenton Tarrant grew up in Grafton in NSW and had said in his own "manifesto" he had planned to carry out an attack for two years and decided to target Christchurch three months ago.

Two other men and a woman were also arrested after at least one gunman opened fire as hundreds of worshippers gathered for Friday prayers.

One man, in his late 20s, had been charged with murder, Police commissioner Mike Bush said.

He declined to say if Tarrant was the person charged.

Bush confirmed at least 49 people had been killed, while car bombs were also disengaged by military personnel after the shooting.

About another 40 were still in Christchurch hospital, ranging from children to adults, being treated for gunshot wounds.

"It is clear that this can now only be described as a terrorist attack," New Zealand prime minister Jacinda Arden said.

"From what we know, it does appear to have been well planned. Two explosive devices attached to suspects' vehicles have been found and they have been disarmed."

The shooting is the deadliest in New Zealand history and Ms Ardern called it one of her country's darkest days.

The country's terrorism threat level has been raised from low to high for the first time.

"We were not chosen for this act of violence because we condone racism, because we are an enclave for extremism," she said.

"We were chosen for the very fact that we are none of those things. Because we represent diversity, kindness, compassion. A home for those who share our values. Refuge for those who needs it.

"And those values will not and cannot be shaken by this attack."

Ardern said three of those arrested were directly connected to the attack, while the fourth was not, and none had been on security watchlists.

Bush said Australia security agencies had not been tracking them either.

Armed police descended on the Masjid Al Noor mosque, near Hagley Park, after shooting broke out at 1.40pm (NZT) on Friday, with public facilities locked down and the area cleared out.

Reports later emerged of shots at another mosque in Linwood Avenue.

"I can confirm that the individual who was taken into custody I have been advised is an Australian-born citizen," Australian prime minister Scott Morrison told reporters in Sydney on Friday.

A witness, with blood splatters across his shirt, told AAP 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".

"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."

About 200 family members were waiting at the hospital for news on Friday evening.

A 17-minute video taken from a helmet camera showed a shooter in his car, arming himself and getting out at the mosque, had been posted online.

Police urged the public not to share it, and telecommunications companies were working to have it taken down.

A man named as Tarrant and claiming to be a 28-year-old Australian had issued a 74-page "manifesto" saying he carried out the fatal shootings to avenge the deaths of thousands in Europe in terror attacks.

Video posted on social media showed police ramming a vehicle and removing a person inside.

Christchurch Mayor Lianne Dalziel urged residents to pull together.

"I never could believe that something like this would ever happen in the city of Christchurch, but actually I would never believe that this would ever happen in New Zealand," she said.

New Zealand's top national security agencies held an emergency meeting in the afternoon.

Players from the Bangladesh cricket team, in Christchurch to play New Zealand in a Test match starting on Saturday, were in the mosque narrowly escaped the attack. They had been left shaken but uninjured, their coach told media.

The match since been cancelled.

Australian Associated Press