Global security expert Joe Siracusa has called the mass shooting in Christchurch a "catastrophic failure of intelligence".

Four people have been arrested over the shooting, which killed at least 49 people in two mosques.

Australian man Brenton Tarrant is among those arrested, and is alleged to have been a gunman who carried out the attack .

An AOS (Armed Offenders Squad) member is seen following a shooting at the Masjid Al Noor mosque in Christchurch. (AAP)

One of the four people in custody has been charged.

Prof Siracusa told A Current Affair that New Zealand and potentially Australia had questions to answer over how the attack was carried out.

"Every time the terrorists are successful, beginning from 9/11 to the current day, it means there's an intelligence failure," he said.

He said he found it "unbelievable" Mr Tarrant had not been on a New Zealand watchlist.

Alleged gunman Brenton Tarrant. (Supplied)

In Pictures: Aftermath of the Christchurch mosque shooting

"This man seemed to telegraph everything," he said.

He also said if the attack was carried out by a cell rather than a lone wolf, it added to the failings of the authorities.

"This is a catastrophic failure of intelligence," he said.

"And if Australia had a hand in it, they have to explain it too."

The two mosques which were targeted in the deadly attacks in Christchurch, New Zealand. (Google Maps)

Prof Siracusa played down fears the shooting could spark retaliatory attacks, but suggested it could inspire people with similar beliefs.

"These things tend to reverberate," he said.

"Terrorists around the world, whether on the left or right, they love their publicity."

But he said in the US and Australia, authorities were well aware of the threat of white supremacist terrorism.