A witness said many people were killed but police have not yet described the scale of the shooting but urged people in central Christchurch to stay indoors.
New Zealand police say shooter is still active. "A serious and evolving situation is occurring in Christchurch with an active shooter," police said.
"Police are responding with its full capability to manage the situation, but the risk environment remains extremely high."
Local media said there were multiple casualties at one mosque and said another had been evacuated.
Police commissioner Mike Bush said all schools in the city had been placed in lockdown in response to "a serious ongoing firearms incident".
"Police urge anyone in central Christchurch to stay off the streets and report any suspicious behaviour," he said.
An eyewitness told Radio New Zealand he heard shots fired and four people were lying on the ground, with "blood everywhere".
There was no official information on casualties but the New Zealand Herald said at least one person was killed.
ESPN Cricinfo reporter Mohammed Isam said members of the Bangladesh cricket team, who are due to play a Test match in Christchurch tomorrow escaped from the mosque.
"They ran back through Hagley Park back to the Oval," he tweeted.
"Entire team got saved from active shooters!!!" tweeted Tamim Iqbal. "Frightening experience and please keep us in your prayers." (AP, AFP)