A Fire and Emergency NZ spokesman said they were called to the fire at about 1pm.
Members of the public had reported a large fire at a property in Bowhill Rd in New Brighton.
Fire crews arrived to find a two-storey house well-involved in fire.
A reporter at the scene said police are armed and telling people to leave the area near the fire, saying it is an active crime scene.
The road has been cordoned off and about five fire trucks and up to 15 police cars are attending.
A post on the Canterbury Fire and Emergency Facebook page asked people to avoid the area.
"If you are downwind of the incident, please stay out of the smoke and close doors and windows."
People at the scene said they had heard a loud bang, which they believed was an explosion, before coming out to see the building up in flames.
A police spokesperson said there are no reports of any injuries.
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