The cause of Christchurch's devastating Port Hills fire more than seven months ago - remains unknown

650 hectares of the hills went up in smoke, forcing evacuations from 80 properties.

A fire investigation report has found the blaze on February 14, began on private property, but how remain undetermined.

District Commander Dave Stackhouse says the property owner confirmed no activity or equipment was used on the day the fire started.

Police also interviewed three witnesses in the vicinity at the time.

The investigation will re-open if further information becomes available.