The South Island city was placed in lockdown as police hunted for an “active shooter”.
Bangladesh cricket team, who were at the mosque for Friday prayer when the shooting started, escaped unscathed.
“A serious and evolving situation is occurring in Christchurch with an active shooter,” police said in a statement.
Prime Minister Jacinda Ardern said on the mosque shootings were one of New Zealand’s darkest days and that one suspect was in custody.
Ardern, who spoke on television soon after attack said it was an “extraordinary and unprecedented act of violence”.
Witnesses told media that a man dressed in a military-style, camouflage outfit, and carrying an automatic rifle had started randomly shooting people in the Al Noor mosque.