An eyewitness said the shooter in the mosque near Hagley Park was "very calm", and was short, had covered his face and was wearing an army uniform.
Armed police have been deployed after shots were fired at two mosques where hundreds of people were praying in Christchurch.
Forty-nine people have been killed.
An eyewitness at the Masjid Al Noor Mosque near Hagley park during the shooting told Checkpoint the man was "very calm", and was short in stature, had covered his face and was wearing an army uniform.
The gunman entered the back door and started shooting people, firing, reloaded and firing again.
"He started to shoot them, anyone who he thinks is still alive he will continue shooting them.
"He didn't want anyone to stay alive."
The eyewitness hid in the pulpit and people broke the windows and ran away.
Mohammad Jama was also at the mosque. He told Checkpoint the shooter was a short, white man, and he had a long black automatic weapon. There were 250 men in the prayer room, and around 50 women.
"And he opened fire everywhere, everywhere. And the people were shouting and running and running."
Mohammad Jama said he called the police but didn't see where the man went.
An eyewitness describes seeing people covered in blood with more than a dozen people injured after a shooting at mosques in Christchurch.
Watch: a witness describes police apprehending one of the suspects by ramming his car:
Azam Ali - who was praying at Linwood Masjid mosque in Christchurch with about 150 others - said he saw up to 13 people injured after a man walked in and opened fire.
"We were into 10 minutes of our prayers and then we heard gun shots outside, but kept on praying. Next minute, it was inside. He was a light-coloured skin guy and he started firing and we all went for cover.
"A couple of guys from inside probably ran outside and they all came out in blood. When we got up we saw people lying around us [who] were shot. They had blood coming out, some from the neck.
"[There] were between eight to 13 people [injured]. When we didn't hear anymore shots we got up ... five minutes after that the police turned up and escorted us outside.
"I've lived in Christchurch for twenty-five years plus, I haven't seen anything like this. This is not right, this is not Christchurch. Day by day there's so much racism [and] the conflict we are getting, it's not right."
A teacher at a childcare centre close to the the Masjid Al Noor Mosque next to Hagley Park said a parent has told her her father-in-law has been shot in the legs.
Dr Maysoon Salama is the manager at a childcare centre nearby.
She said she's heard from parents that about 30 people have been shot and injured, and that people have been killed.
She'd heard that there's more than one gunman, and that one of the shooters had run away and was being chased by police.
A parent who had come to the centre to pick up her daughters said her father-in-law had been shot in the legs, Dr Salama said.
"We asked about our families but she didn't know."
They were trying to contact the parents to take their children home and it was "chaotic situation".
She said they have locked all the doors and windows.
Dr Salama said she had been trying to get hold of her husband and son but had not been able to yet. She has last heard from them when they left to go to prayers which started at 1.30pm.
"It seems that people who went a little before time were trapped there, people who came just on the dot were outside the mosque and were not allowed to enter because the shooting had already started."
'A few shots, then some more, then some more'
Andrew Read was in an office about 200m from the Masjid Al Noor Mosque on Deans Avenue in Christchurch.
His is office was in lockdown, and armed offenders are going through his building and buildings next door.
He and his colleagues heard the gunshots.
"It sounded like a semi-automatic weapon, so very regular shots going off, and quite a number, there's been reports of 20 - it was easily 20 shots.
"A few shots, then some more, then some more, and it was quite awhile before we heard sirens, and realised something was going on."