Four people are in custody - three men and one woman.
Prime Minister Jacinda Ardern has called the mass shootings a terror attack.
Ali, 62, had been attending regular Friday prayers at the mosque, and said there were around 300 worshippers in the building at the time. He said the sermon was being given when the shooting started.
"I saw people running for all the doors. To get 300 people out of the doorways was not easy, out of two doorways, because he entered through the main door, and there were two more doors on the sides."
He dived behind a bench, although he said his legs would have been visible.
"He just started shooting 'Bang, bang, bang'. My cousin was sitting beside me and he got hit in his leg."
The shooting stopped and restarted seven times as the shooter kept reloading ammunition, he said.
"I'm thinking, 'Oh my God, I hope he finishes his bullets. I hope he finishes them'."
Ali said he decided to make a dash for it. Another man was sitting near the main door and gestured for him to come towards him.
"And what [the shooter] did, he shot him on his bloody chest. Right in front of me."
He dived through an already-broken window and then into Hagley Park.
"I was the last person to get out of the mosque alive."
Ali said his brother Ashraf was in the mosque and was still missing.
"When I looked back there were people dead on both sides of the door. I couldn't see my brother. I just hope he's alive."
His cousin's husband and a friend were also badly injured.
Originally from Fiji, Ali moved to New Zealand in 1989 and shifted to Christchurch five years ago. He works as a halal food auditor.
"People are so good here, when I am around doing my work. They have always been so friendly, very friendly. Even [after the shooting] they came to the cordon with tea and coffee."
Police set up missing persons website to help public find loved ones
Police are also making an effort to inform the public of the Restoring Family Links (RFL) website where people can register missing persons or register themselves as alive.
The Red Cross has been asked by police to activate the website due to today's massacre in Christchurch.
The RFL website is a tool where the public can self-register they are alive so family and friends know they are safe. Members of the public can also register missing persons.
People living in New Zealand can also register missing persons on 0800 115 019, and a member of police will get in touch with you.
