The blaze on Christchurch's Port Hills in February started on private property but investigators have not determined how it started, a new Fire and Emergency New Zealand report has found.
The fire started at the property near the Worsley Track on February 14 and spread over 700 hectares, forcing the evacuation of more than 100 properties.
The blaze triggered a major firefighting operation including more than 100 firefighters, 15 helicopters, two fixed-wing aircrafts and almost 30 fire engines.
A state of emergency was declared by the Christchurch City and the Selwyn District Council and remained in place for a week.
The report stated the cause of the fire could not be proven to an acceptable level of certainty, therefore it was classified as "undetermined".
Fenz incident commander Dave Stackhouse said investigators pinpointed a specific origin area of about five square metres, about 50 metres off the side of Worsleys Rd.
"The owner of the property where the fire originated co-operated with the investigation and advised that there had been no activity or equipment used on the day the fire started," he said.
"Our legal advice is that, due to privacy reasons, Fire and Emergency is unable to release the specific origin area of the fire because it is on private property."
Stackhouse said the outcome could be disappointing for everyone affected by fire who were hoping for answers.
"Our investigation into the cause of the fire included three experienced wildfire investigators working alongside the police and who arrived in Christchurch the day after the fire started," he said.
Investigators interviewed three witnesses who were in the vicinity of the fire when it started and were the first to alert emergency services.
The report said the combination of "dry weather, low moisture content and a continuous available fuel load" helped the fire ignite and its subsequent spread.
"These conditions assisted in the ignition and spread of the fire," he said.
While the cause of the fire was unclear, Stackhouse said significant fires such as the Port Hills blaze sent a strong reminder to the public to protect themselves from wildfire.
"Now is the time to prepare for the fire season," he said.
"We encourage the community to develop an emergency plan, this should include a plan for pets and livestock
"We also recommend that property owners take action to create a defensible space around their properties, remove unwanted vegetation and leaves and use low flammability plants to assist with creating a green fire break around properties."
The fire burned for three weeks, scorching about 650 hectares, destroying a house and other infrastructure and damaging the Christchurch Adventure Park.
The property owner cooperated with the investigation and advised that there had been no activity or equipment used on the day the fire started, Stackhouse said.
"As the cause of the fire cannot be proven to an acceptable level of certainty, it is classified as undetermined. However, if further information becomes available, the investigation will be reopened."
Legal advice was that for privacy reasons Fenz was unable to reveal the area the fire started, because it was on private property.
"We acknowledge this may be disappointing for all those who were affected by the Port Hills fire in February and were hoping for answers," Stackouse said.
