SINGAPORE - One of the gunmen in the Christchurch mosque attacks worked as a personal trainer at a gym in the northern New South Wales city of Grafton, Australian Broadcasting Corp online reported on Friday (March 15).

The gunman livestreamed his assault on the Al Noor mosque in central Christchurch and posted a lengthy manifesto on Twitter afterwards giving his reasons for the attack. Media have identified the man as 28-year-old Brenton Tarrant.

New Zealand authorities have said three people are in custody for the attacks and that a man in his late twenties has been charged with murder but they have not named him or given any other details of the suspects.

Australian Prime Minister Scott Morrison confirmed one of the attackers was Australian born.

Tracey Gray, manager of the Big River Gym, confirmed the man who filmed the attack and streamed it online was Tarrant, ABC said.

She said he worked at the gym after he finished school in 2009 until 2011, when he left to go travelling overseas in Asia and in Europe.

"He was a very dedicated personal trainer," Ms Gray told ABC.

"He worked in our program that offered free training to kids in the community, and he was very passionate about that."

Ms Gray said Tarrant did not strike her as someone who had an interest in firearms.

"I think something must have changed in him during the years he spent travelling overseas," she said.

Tarrant is known to have visited Europe, South-East Asia and east Asia.

Tarrant inherited his late father's interest in physical fitness.

"He was very good, very dedicated with his training' Ms Gray told ABC. "He would train a lot, and some could say quite excessively but then he was passionate about health and fitness and making those changes in his personal space."

Ms Gray said Tarrant was in every way a model fitness instructor who went out of his way to help people and put in extra effort.

"I honestly can't believe that somebody I have probably had daily dealings with and had shared conversations and interacted with would be capable of something to this extreme."

Other Australian media outlets also said Tarrant was from Grafton.

Family members who remain in the Grafton area were in shock and trying to come to terms with his acts, and are assisting police with their investigations, Sky News said.

In April 2010, his father died suddenly of cancer aged 49, leading Tarrant to set off on a seven-year trip around the world, Sky News said.