Update, March 15, 4:50pm: Death toll updated.

New Zealand Prime Minister Jacinda Ardern has characterised Friday's mosque shootings as a terrorist attack during an afternoon news conference.

Latest updates

Some 49 people have reportedly been killed after a gunman opened fire inside two mosques -- 30 deaths were reported in one mosque.

The shooting attack took place on the southern island of New Zealand in the city of Christchurch.

Another 20 people suffered serious injuries.

Police have apprehended four suspects -- three men and a woman.

A number of bombs were detected and neutralised on the attackers' automobiles.

Reports of improved explosive devices (IEDs) strapped to vehicles have been secured.

"Violent terrorist"

Arden made reference to the nationalities of the victims, saying: "Many of those directly involved might be immigrants, refugees who chose to be here. They are us. The person who has done this, is not."

She also said the four people were not on any active terrorist watch lists and it was "not a matter of someone slipping under the radar".

Australian Prime Minister Scott Morrison confirmed one of the four suspects was an Australian citizen.

Morrison described the shootings as having been perpetrated by an "extremist, right-wing, violent terrorist, that has taken the lives -- stolen the lives -- in a vicious, murderous attack that has claimed so many New Zealanders."

What happened

Witnesses said the attack occurred just before 1:40pm New Zealand time as the worship leader gave the sermon.

Gunmen entered two mosques and began shooting.

One gunman was seen dressed in black with a helmet carrying a machine gun.

He entered the premises with an automatic weapon and began firing at the people praying at Masjid Al Noor mosque -- which suffered heavier casualties.

There were about 50 people inside the building.

NZ Police Commissioner Mike Bush said four suspects were in custody, he added: "Let's not presume the danger is gone."

"... one of New Zealand's darkest days"

Police confirmed a second shooting occurred at the Linwood mosque during Friday prayers in the South Island city, but no details were immediately available.

"We're dealing with an evolving situation across multiple sites," Ardern said at a news conference. "This is, and will be, one of New Zealand's darkest days."

New Zealand media initially reported between nine and 27 people were killed, but the death toll could not be confirmed.

Police warned worshippers not to visit mosques "anywhere in New Zealand".

A lockdown imposed throughout Christchurch has been called off hours after they were in place following the attacks.

One gunman fled before emergency services arrived.

Livestreamed attack

One of the gunmen shared a livestream of the attack on Facebook and posted content on Instagram.

It lasted for about 18 minutes.

Facebook said it has taken down the video and was removing praise for the gunman.

"Police are aware there is extremely distressing footage relating to the incident in Christchurch circulating online," a police statement said. "We would strongly urge that the link not be shared. We are working to have any footage removed."

There were reports of racist literature was left behind at the scene denouncing "invaders".

A manifesto by one gunman is also making its rounds online.

The gunman had reportedly said in the first few minutes of the stream: "Remember lads, subscribe to Pewdiepie."