Witnesses told local media there were several casualties inside the building. Police have warned people to avoid the area.
Christchurch hospital and all of the city's schools have been placed on lockdown.
Eyewitnesses have reported running for their lives to escape a shooter.
Mohan Ibrahim 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."