The head of the Federation of Islamic Federations of New Zealand, Dr Mustafa Farouk, told RNZ that up to 500 people were inside the mosque.
The man said he initially planned to target a mosque in Dunedin, but changed to the Al Noor Mosque because it had "far more invaders".
Apart from numerous witness reports of casualties, a reporter following the Bangladesh cricket team, who are now in New Zealand, tweeted that the team had "escaped from a mosque near Hagley Park where there were active shooters".
The New Zealand Herald reports that the incident took place at a mosque, and that there were numerous casualties.
Eye witnesses say they've seen about six people taken into Christchurch Hospital, two with serious injuries.
Bush urged anyone who had meant to visit a mosque in New Zealand not to go, to "close your doors until you hear from us again".
"Police are responding with its full capability to manage the situation, but the risk environment remains extremely high". However, Ardern said other individuals could be involved in the attack and cautioned Christchurch residents to remain sheltered and follow the instructions of local authorities.
"Many of those who would have been affected by this shooting may be migrants to New Zealand", Ardern said. Muslims account for just over 1 percent of New Zealand's population, a 2013 census showed.
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Australian channel Nine News Melbourne has reported that there may be as many as 27 deaths in the twin mosque firings in Christchurch. I don't understand how anyone could do this to these people, to anyone.
"This is not who we are". Over the course of five minutes, he repeatedly shoots worshippers, leaving well over a dozen bodies in one room alone. "The persons who has perpetuated this violence against us ... have no place in New Zealand".
Peneha says he went into the mosque to try and help: "I saw dead people everywhere". Students participating in the city's Fridays for Future climate march were confirmed to be safe, Mayor Lianne Dalziel told the New Zealand site Newshub.
He says he also saw the gunman flee before emergency services arrived. He added, "It's unbelievable nutty".
"So I tried to get out and I pushed myself at the back where my vehicle was, and from there I heard the shooting, it went on six minutes or more".
The Bangladesh cricket team is in Christchurch to play New Zealand in a third cricket test starting on Saturday.
The gunman live-streamed the mass shooting inside the Al Noor Mosque, which happened about 1.30pm (11.30am AEDT) as Friday prayers were underway.
Police have not confirmed a specific number of casualties but are warning people to stay indoors until the situation has resolved and to report any suspicious information to police.