Mr Nabi said he was approaching one of the mosques with his 8-year-old daughter when he learned about the attacks from police who were telling people to avoid the area.
New Zealand Police Commissioner Mike Bush confirmed two of those in custody were arrested at the scene, and that police were working to establish whether they had had any involvement in the incident.
All of the guns Tarrant used during the attack were purchased legally, according to gun laws and restrictions in New Zealand.
She said the man was arrested by two "rural community cops" 36 minutes after the first emergency call was made on Friday and was still an active shooter.
The Super Rugby match between the Otago Highlanders and Canterbury Crusaders scheduled for Saturday has been cancelled after the terrorist attack on two mosques in Christchurch on Friday killed 49 people.
Footage of the attack live streamed on Facebook showed the gunman entering the mosque and shooting randomly at people.
The winners of New Zealand's first-class domestic cricket competition, Plunket Shield, has been decided with a round left, in the wake of the terrorist attack at a mosque in Christchurch.
During the hearing, Judge Paul Kellar in the Christchurch District Court said it was "reasonable to assume" that the man would face further murder charges. "Rhetoric of racism, division and extremism has no place not only in New Zealand but I would say in a society as a whole".
Police said they were not searching for any other suspects but stressed the investigation remained fluid.
In 2014, police estimated there were up to 1.2 million legal firearms in civilian ownership, or around one for every four members of the public - twice the per capita number of guns in Australia.
Meates said at a news conference at the Christchurch hospital that it's "hard to fathom the enormity of this act of terrorism".
"This is always the biggest fixture on our match calendar and one which our fans look forward to", said Clark, who would have been expecting a crowd of some 20,000.
Mr Bush, who had earlier called the attack a "very well-planned event", said the suspect was not known to police either in New Zealand or Australia. In fact, for many, New Zealand was a choice, a place they actively came to and committed themselves to, the place they were raising their families, where they were part of communities they loved and who loved them.
New Zealand's stricken residents reached out to Muslims in their neighbourhoods and around the country on Saturday.
The attack has prompted an outpouring of grief and deep shock in this usually peaceful and hospitable country, which prides itself on welcoming refugees fleeing violence or persecution.