Christchurch's Port Hills fire started on private property but fire investigators have not determined how it broke out, according to a Fire and Emergency report into the blaze.
The fire started at the property near the Worsley Track on 14 February 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 blaze destroyed a container home.
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 Road.
"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.
They assessed photos and videos of the early stages of the fire which were sent in by the public or captured on CCTV, Stackhouse said.
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."
FENZ incident commander Dave Stackhouse. Photo: RNZ
Stackhouse said despite a lengthy investigation and the source being located, the cause of ignition could not be determined.
"If we establish a cause sufficiently, it can go towards prevention of fire because we can put strategies in place around that. Obviously New Zealand Police can also help if we have patterns of suspicious fire, and if we have particular causes, we can look at mitigation to eliminate them."
He said more eyewitness accounts, video and photos would be useful, and Canterbury Police district prevention manager Inspector Ash Tabb said forensic evidence would also be helpful.
"We do have a number of CCTV footage, we've done a lot of work with the witnesses which have come forward, "Tabb said. "We probably need additional information from other witnesses or… further photographic evidence."
Tabb said there were always a lot of people on the Port Hills at any one time.
Stackhouse said Fire and Emergency had worked with community groups to inform locals about low-flammable species and "defendable spaces" around residential properties.
They were also working closely with NIWA to know what kind of weather conditions lay ahead.
"What I'd like to see is quite a lot of heavy rain," he added, noting NIWA had forecast a hot and dry summer.
John Filsell, council head of community support and partnerships, said by the end of the year there would be a "Port Hills plan process" to "provide and idea forum to have those wider discussions on what's planted on the Port Hills and why".
"Considerations do have to be balanced against what private property owners can do quite legitimately and legally on their own land."
