Three men and one woman are in custody, police commissioner Mike Bush told a news conference.
Witnesses told local media they ran for their lives, and saw people bleeding on the ground outside the Al Noor mosque.
Authorities advised all mosques to shut down until further notice. All Christchurch schools are on lockdown.
In a statement to the nation, Prime Minister Jacinda Ardern said details were not yet clear, “but I can tell you now – this will be one of New Zealand’s darkest days”.
Mohan Ibrahim, who was in the area of the Al Noor mosque, told the New Zealand Herald: “At first we thought it was an electric shock but then all these people started running.
“I still have friends inside.
“I have been calling my friends but there are many I haven’t heard from. I am scared for my friends lives.”
It is not yet known how many shooters there were, but the Herald reports that one gunman is believed to be an Australian who has written a manifesto outlining his intentions. In it, he espouses far-right ideology and anti-immigrant ideology.
Unverified footage purportedly taken by the shooter has emerged, suggesting he filmed as he shot victims. Police called on the public not to share the “extremely distressing” material online.