The fire service was called to two suspicious fires in the same building in the same street in Christchurch last night.
Witnesses have told police they saw two people fleeing the scene on Hereford Street last night, and Detective Sergeant Craig Farrant said it was obvious that it was an arson attack.
"An intentionally lit fire at about 4.30 yesterday afternoon at 139 Hereford Street and then again at the same location, it was lit again in a different location within the house, at about 8.30 yesterday evening."
Detective Sergeant Farrant said it was too early to release descriptions of the people at this stage.
Assistant Area Commander Dave Burford said the fires on Hereford Street were in an empty property.
"We're very concerned about unoccupied houses getting set alight and the police are handling all matters."
He is urging residents to be vigilant at all times and report any suspicious activity to the police.
A third suspicious fire was at a property at Pines Beach.
Assistant Area Commander Dave Burford is very concerned about the recent spate of fires.
"We just asked the people to keep vigilant and be security-conscious and reporting suspicious activities to the police."
It was too early to draw any conclusions about exactly how the fires started, he said.