Facebook said Friday it had "quickly" removed a live video from the suspected gunman in twin mosque shootings that killed at least 49 people in New Zealand.

The 28-year-old shooter, who has been arrested, published a racist manifesto on Twitter before live streaming his rampage showing him repeatedly shooting at worshipers from close range.

"Police alerted us to a video on Facebook shortly after the live stream commenced and we quickly removed both the shooter's Facebook and Instagram accounts and the video," the social network said.

"We're also removing any praise or support for the crime and the shooter or shooters as soon as we're aware."

The Facebook Live video, taken with a camera that appeared to be mounted on the gunman's body, shows a clean-shaven, Caucasian man with short hair driving to the Masjid al Noor mosque in central Christchurch.

He enters the building and fires repeatedly at worshipers as he moves from room to room.

AFP determined the video was genuine through a digital investigation that included matching screenshots of the mosque taken from the gunman's footage with images available online showing the same areas.