An Australian man accused over the massacre of at least 49 people at two Christchurch mosques had apparently posted his plans online the day before the attack.
Former NSW personal trainer Brenton Tarrant is one of four people arrested over the shootings as hundreds of worshippers gathered for Friday prayers at mosques in the New Zealand city.
Tarrant, who grew up in Grafton in NSW, stated in a rambling 74-page "manifesto" he had spent years planning an attack in vengeance for deaths in Europe, deciding on the South Island city three months ago.
A post on Thursday on a message board website linked to Tarrant also said the attack "against the invaders" would be livestreamed on Facebook.
Two other men and a woman were also arrested.
Police said one of the men aged in his late 20s was charged with murder and will appear in Christchurch court on Saturday, but would not specifiy whether it was Tarrant.
Police said 41 people were killed at the Masjid Al Noor mosque on Deans Avenue, seven in a second six kilometres away at the Linwood Masjid, and one more in hospital.
Another 40, ranging from children to adults, were being treated for gunshot wounds, hundreds of family members waiting for news in Christchurch Hospital
Army personnel were later called in to dismantle explosive devices found in a stopped car and police were in the evening searching a house in Dunedin, 360 kilometres away, clearing nearby homes for safety.
None of the four arrested, three directly tied to the attack, were on the watchlists of New Zealand or Australian security agencies and their alleged movements and involvements during the shootings are still unclear.
Prime Minister Jacinda Ardern said the well-planned attack - the worst shooting in the country's history - would be treated as terrorism, and New Zealand's threat rating was moved to high for the first ever.
But she refused to accept it would change her nation's values.
"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 need it.
"And those values will not and cannot be shaken by this attack."
Reports of gunfire first came at about 1.40pm on Friday, armed police descending on the Al Noor mosque near Hagley Oval, clearing the public from the area and sending the city into lockdown.
Reports of a shooting at the second mosque followed, before video emerged of police ramming a car and pulling out the occupant.
Witnesses described bloody scenes and bodies falling to the ground as worshippers ran for doors and a shooter moved from room to room for around 20 minutes.
One told AAP he saw the gunman reload seven times.
"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."
In the manifesto, Tarrant described himself as "Just an ordinary White man, 28 years old. Born in Australia to a working class, low income family".
"I only arrived to New Zealand to live temporarily whilst I planned and trained, but I soon found out that New Zealand was as target rich of an environment as anywhere else in the west," he wrote.
A 17-minute livestreamed video taken from a helmet camera showed a shooter in his car, arming himself, getting out of the vehicle and entering the mosque where he started shooting appeared online.
Authorities and telecommunications companies were scrambling to take it down on Friday.
With Christchurch citizens in shock and mourning, Mayor Lianne Dalziel urged her city to come together.
"This tragedy will touch all of us. No one will be unaffected," she said.
"We will all need to pull together and support each through the coming days and weeks as we come to terms with the immense loss that our city has experienced."