Investigations into Christchurch's Port Hills fire have uncovered the where, but not the how

650 hectares went up in smoke, forcing evacuations from 80 properties in mid-February.

A fire investigation report's identified the small area where the blaze burst out, but its cause remains undetermined.

District Commander Dave Stackhouse says the flames ignited on private property.

He says investigators spent a lot of time narrowing down the area of origin, but haven't been able to establish the ignition source.