Accounts emerged on Sunday of heroic attempts to tackle a gunman who slaughtered 50 people at two mosques in New Zealand, as authorities prepared to begin releasing the bodies of victims to their families for burial.

Australian Brenton Tarrant, 28, a suspected white supremacist, was charged with murder on Saturday. Tarrant was remanded without a plea and is due back in court on April 5 where police said he was likely to face more charges.

Friday's attack in the city of Christchurch, which Prime Minister Jacinda Ardern labelled terrorism, was the worst ever peacetime mass killing in New Zealand.

Ardern also said she wanted to talk to Facebook, as footage of the attack on one of the mosques was broadcast live on Facebook, and a "manifesto" denouncing immigrants as "invaders" was posted online via links to related social media accounts minutes before the violence began.

Amid the shock, outrage and recriminations that have consumed New Zealand over the past two days, tales of heroism and self-sacrifice emerged.

Abdul Aziz, 48, was hailed for confronting the shooter at the second mosque and preventing more deaths.

Aziz, originally from Afghanistan, ran outside after the shooting started and picked up a shotgun that the gunman had dropped. The gun had no shells in it, he said.

"I chased him," Aziz said. "He sat in his car and with the shotgun in my hands, I threw it through his window like an arrow. He just swore at me and took off."

Another man, Naeem Rashid from Pakistan, was seen on the gunman's video confronting the shooter before he was killed, the BBC reported. Rashid's 21-year-old son, Talha, was also killed.

The death toll climbed to 50 when police found another body at the Al Noor mosque, where more than 40 people died after a gunman burst in and opened fire on worshippers with a semi-automatic rifle with high-capacity magazines, driving to attack a second mosque.

Police rammed the suspect's vehicle and arrested him as he drove away from the second mosque in the suburb of Linwood.

Police Commissioner Mike Bush said the man was apprehended 36 minutes after police were alerted and he was the only person charged in connection with the shootings. Three people detained earlier were not involved, he said.

Huge piles of flowers were laid at sites near the mosques and crowds of people of all faiths gathered to pay respects. Some played guitar, sang and lit candles as darkness fell.

Members of a Maori motorbike gang performed a haka war-dance at one site.

Church services for the victims were held, including at Christchurch's "Cardboard Cathedral", a temporary structure built after a 2011 earthquake.

But the priority for grieving family and friends on Sunday was laying their loved ones to rest. It is customary in Islam to bury the dead within 24 hours but no bodies have been released because of the investigation, police said.

Ardern said victims would be handed over to families from Sunday evening.

"It is likely, however, to be a small number to begin with," she told a media briefing, adding that all should be returned by Wednesday.

Wearing a black scarf, Ardern hugged members of the Muslim community at a refugee centre on Saturday, saying she would ensure freedom of religion.

Thirty-four people were in Christchurch Hospital, with 12 in intensive care, while a child was moved to a children's hospital in Auckland.

Greg Robertson, head of surgery at Christchurch Hospital said staff were used to gunshots and other severe injuries, but the scale and nature of the attacks was different.

"The magnitude of this is the thing that is the most significant issue for people. It's just comprehending what is the incomprehensible."

The majority of victims were migrants or refugees from countries such as Pakistan, India, Malaysia, Indonesia, Turkey, Somalia, Afghanistan and Bangladesh.

The youngest was a three year old boy, according to an unofficial list of the dead.

Pakistan's high commissioner said six citizens had been killed and three were missing. Five Indians were killed, its High Commission said.

Tarrant did not have a criminal history and was not on any watchlists in New Zealand or Australia.

Ardern said a "manifesto" was emailed to more than 30 recipients including her office, nine minutes before the attack but it gave no location or specific details. She said her office sent it to parliamentary security two minutes after getting it.

In the manifesto, which was also posted online, Tarrant described himself as "Just a ordinary White man, 28 years old" who used profits from cryptocurrency trading to finance travels through Europe from 2016-2018.

The shootings have raised new questions about violence being disseminated online. Facebook said it had removed 1.5 million videos of the attack in the first 24 hours, and it was also removing all edited versions, even those without graphic content.

Ardern told the briefing that she had been contacted by Facebook operations chief Sheryl Sandberg who had acknowledged what had happened.

"This is an issue that I will look to be discussing directly with Facebook," Ardern said.

The violence has also shone a new light on gun control.

Ardern said Tarrant was a licensed gun owner who allegedly used five weapons, including two semi-automatic weapons and two shotguns, which had been modified. She said a ban on semi-automatic weapons would be considered.

"We cannot be deterred from the work that we need to do on our gun laws," Ardern said. "They will change."

Media has reported a rush to buy guns before any ban is brought in.

New Zealand has tried to tighten laws before but a strong gun lobby and culture of hunting has stymied efforts.

There are an estimated 1.5 million firearms in New Zealand, which has a population of only 5 million, but it has had low levels of gun violence.