Updated 9.15am

Attacks on two Christchurch mosques left at least 49 dead Friday, with one gunman – identified as an Australian extremist - apparently livestreaming the assault that triggered the lockdown of the New Zealand city.

In what appeared to be the worst attack against Muslims in a western country, witnesses spoke of victims being shot at close range, with women and children believed to be among those killed.

"It is clear that this can now only be described as a terrorist attack," said Prime Minister Jacinda Ardern, saying it marked "one of New Zealand's darkest days".

"From what we know, it does appear to have been well planned," she said, adding that in addition to the dead another 20 people were seriously injured.

The gunman at one mosque was an Australian-born citizen, Australian Prime Minister Scott Morrison said in Sydney, describing him as "an extremist, right-wing, violent terrorist".

It was not immediately clear how many attackers were involved, but Ardern said three men had been taken into custody.

Two IEDs (improvised explosive devices) were also found and neutralised by the military, police said.

A Palestinian man who was in one of the mosques said he saw someone being shot in the head.

"I heard three quick shots, then after about 10 seconds it started again. It must have been an automatic -- no one could pull a trigger that quick," the man, who did not wish to be named, told AFP.

The gunman at one mosque was an Australian-born citizen, Australian Prime Minister Scott Morrison said in Sydney, describing him as "an extremist, right-wing, violent terrorist".

It was not immediately clear how many attackers were involved, but Ardern said three men had been taken into custody.

Two IEDS (improved explosive devices) were also found and neutralised by the military, police said.

A Palestinian man who was in one of the mosques said he saw someone being shot in the head.

"I heard three quick shots, then after about 10 seconds it started again. It must have been an automatic - no one could pull a trigger that quick," the man, who did not wish to be named, told AFP.

"Then people started running out. Some were covered in blood," he said, adding that he joined the fleeing crowd and managed to escape.

Local media reported at least nine people were dead.

Video and documents circulating online - but not officially confirmed - suggested the shooter had streamed his attack on Facebook Live.

AFP has examined the footage, which has subsequently been taken down. Journalists experienced in verification techniques said it appeared to be genuine.

New Zealand police described it as "extremely distressing" and urged web users not to share it.

A manifesto had also been posted online on accounts linked to the same Facebook page, suggesting the attack was racially motivated.

A number of pictures were posted to a social media account of a semi-automatic weapon covered in the names of historical figures, many of whom were involved in the killing of Muslims.

Police, who initially imposed a city-wide lockdown, sent armed officers to a number of scenes.

'Darkest days'

An ashen-faced Ardern told reporters the attacks had been "an extraordinary and unprecedented act of violence".

"It is clear that this is one of New Zealand's darkest days," she said.

The two known targets were the Masjid al Noor in central Christchurch, and a second mosque in suburban Linwood.

One witness told stuff.co.nz he was praying when he heard shooting - and then saw his wife lying dead on the footpath outside when he fled.

Another man said he saw children being shot.

"There were bodies all over," he said.

An eyewitness told Radio New Zealand that he heard shots fired and four people were lying on the ground, with "blood everywhere".

Read: New Zealand's Muslim community targeted in attack

Police warned Muslims all over the country not to visit mosques "anywhere in New Zealand". Friday is Islam's holy day.

Christchurch city council offered a helpline for parents looking for kids attending a mass climate change rally nearby.

The Bangladesh cricket team - which had been in Christchurch for a test match against New Zealand that was later cancelled - all escaped without injury.

A spokesman said the attack happened as some of players got off a team bus and were about to enter the mosque.

"They are safe. But they are mentally shocked. We have asked the team to stay confined in the hotel," he told AFP.

Mass shootings are rare in New Zealand, which tightened its gun laws to restrict access to semi-automatic rifles in 1992, two years after a mentally ill man shot dead 13 people in the South Island town of Aramoana.

However, anyone over 16 can apply for a standard firearms licence after doing a safety course, which allows them to purchase and use a shotgun unsupervised.

Christchurch, a relatively small city in the south of New Zealand, hit global headlines in 2011 when it was struck by a deadly earthquake.

Dozens of people died and the city's historic cathedral was toppled in the disaster.

Australia takes precautions

The gunman who killed the worshippers was a right-wing "terrorist" with Australian citizenship, Prime Minister Scott Morrison said.

"We stand here and condemn absolutely the attack that occurred today by an extremist, right-wing, violent terrorist," Morrison told a press conference.

He confirmed media reports that the gunman who mowed down worshippers in two mosques in the southern New Zealand city of Christchurch was an Australian-born citizen.

He said Australian authorities were investigating any links between the country and the attack, but declined to provide further details about the gunman.

Australian media reported the gunman came from a town in New South Wales, of which Sydney is the capital, and state police later said they were increasing patrols around NSW mosques in light of the Christchurch attacks.

The police described the move as a precaution as "there is no ongoing or specific threat to any mosque or place of worship" in the state.

An emotional Morrison offered his sympathies to Kiwis, and announced that flags in Australia would be flown at half-mast in memory of the victims.

"We are not just allies, we are not just partners, we are family," he said.

New Zealand police detained three men after the attacks, but have not provided their identities.

"It is such a sad and devastating reminder of the evil that can be ever present about us," Morrison said of the attacks.