WELLINGTON: New Zealand police detained four people on Friday after mass shootings at two mosques that left 40 people dead in the city of Christchurch.

"Four are in custody. Three are men and one is a woman," New Zealand Police Commissioner Mike Bush told reporters in Wellington, Reuters reported.

Australian Prime Minister Scott Morrison said in Canberra that the lead gunman was a right-wing extremist with Australian citizenship, AFP reported.

New Zealand Prime Minister Jacinda Arden said later there were 40 confirmed dead in the attacks;

"It is clear that this can now only be described as a terrorist attack," she said, 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. 

Police warned against sharing "distressing" video footage relating to the deadly shooting. A video online showed a gunman filming himself firing at worshippers inside a mosque.

The attack was streamed live.

AFP analysed a copy of a Facebook Live video that shows a clean-shaven, Caucasian man with short hair driving to a mosque, then shooting as he enters the building. 

The gunman continues to shoot at people inside the mosque, some of whom were trying to flee while others were huddled in corners of the building, according to the copy of the video that AFP found on YouTube.

A lengthy manifesto posted on the same Twitter account detailed racial motivations for the attack.

The Facebook account that posted the video was no longer available shortly after the shooting. The Twitter account of the same name was quickly suspended.