"New Zealand's darkest day": Everything we know about the Christchurch shooting.

Written with AAP.

Australian Prime Minister, Scott Morrison has confirmed that New Zealand authorities have taken into custody an Australian-born man who was allegedly involved in the Christchurch mosque shooting, which has killed dozens.

Speaking to media, he did not disclose the man’s identity and said he was not previously known to Australian authorities.

“We stand here and condemn absolutely the attack that occurred today by an extremist, right-wing, violent terrorist,” he said.

In his press briefing Morrison shared that Australian agencies including the Australian Federal Police and ASIO are working with New Zealand authorities and that he has been in contact with both the New Zealand Prime Minister Jacinda Ardern.

The Prime Minister also shared his sympathies to those affected - from the victims, to the police officers who responded to the attack.

"I particularly want to express my sincere prayers and thoughts for those New Zealanders, indeed Australians of Islamic faith today, who have been the subject of this callous right-wing extremist attack," he said.

"As the son of a police officer, I can only respond with heartfelt sympathies for the families and for those who have been injured."

Earlier today, Australia's Leader of the Opposition, Bill Shorten also shared a brief Twitter statement regarding the attacks.

"Australia and New Zealand are family and we grieve with our brothers and sisters today," he wrote.

"We send our love and condolences, our sorrow and solidarity in this terrible time of fear and pain and grief."

Although this has not been confirmed by officials, an Australian-born citizen named Brenton Tarrant has been possibly linked to the shootings.

The 28-year-old shared a 37-page manifesto on his Facebook page in the hours before the Christchurch attack began.

Four people in police custody, says New Zealand Police Commissioner, Mike Bush

Four people have been placed in custody and multiple improvised explosive devices (IEDs) have been safely secured by police, says New Zealand Police Commissioner, Mike Bush.

Speaking to the media about today's mosque shooting in Christchurch, New Zealand, Bush did not share the identity of the four people in custody but says police have apprehended three men and one woman.

Referring to the IEDs, the Police Commissioner says that while they have been "made safe by the defence force," this "goes to the seriousness of the situation," he said.

"New Zealand's darkest days," says New Zealand Prime Minister, Jacinda Ardern

Earlier today, New Zealand Prime Minister, Jacinda Ardern addressed the attack which has been reported as New Zealand's worst shooting in history, with some media outlets putting the death toll at over 20 people.

Ardern called the attack an "unprecedented act of violence," however she didn't confirm the number of deceased or injured or the details of the individual who has currently been apprehended by police.

"I'm not currently in a position to confirm the number of deaths or injuries. But I have to acknowledge, people can see images live from Christchurch, it will be obvious to them that this is a significant event," she said.

"This is and will be one of New Zealand's darkest days.

"For now my thoughts and I'm sure the thoughts of all New Zealanders are with those who have been affected their families," she said.

According to early reports, a gunman opened fire at a mosque in New Zealand as people gathered for prayers, sparking a hunt for an "active" shooter in the South Island town of Christchurch.

Armed police were deployed in the city at the Masjid Al Noor mosque where the shooting happened in Hagley Park at 1.40pm (NZT) on Friday and schools across the city are now in lockdown.

"A serious and evolving situation is occurring in Christchurch with an active shooter," Police Commissioner Mike Bush said.

Gunfire was heard coming from the mosque and several people were seen on the ground outside the building, One News reported.

Several hundred people were inside and witnesses reported seeing at least four people on the ground and "blood everywhere", according to Radio New Zealand.

"I heard a big sound of the gun. And a second one, I ran. Lots of people were sitting on the floor. I ran behind the mosque," he told TVNZ.

"The floor. There's lots of blood on the floor, you can see when you go in."

The man, who did not give his name, said he saw four people including a woman on the floor but did not see the shooter.

Christchurch schools, the hospital and the city's university are on lockdown and residents have been told to remain indoors and report suspicious behaviour.

Latest reports say 40 people have been killed, and at least 25 are in critical condition.