New Zealand police commissioner Mike Bush said in a Facebook video there had been "multiple fatalities, as far as we know, at two locations."
He named the two locations as mosques in Christchurch, but added he didn't know if there were other areas under threat.
"We are dealing with a very serious and tragic series of events," he said. "They involve an active shooter, they involve multiple fatalities."
Bush said they had one person in custody but didn't know if there were other people involved.
"I want to ask anyone that was thinking of going to a mosque anywhere in New Zealand today not to go, to close your doors, until you hear from us again," he said.