Christchurch Port Hills fires: Arson or not?

Fire investigators have still not determined if two fires on the Port Hills are arson or not.

The two fires became one when they joined on the Port Hills after starting on Early Valley Rd, Lansdowne and Marley Hill on February 13.

It tore through more than 1645ha and forced the evacuation of more than 1000 residents, claimed nine houses and the life of helicopter pilot Steve Askin.

Rural fire Christchurch manager Tim Mitchell said the fire had burnt back on itself in places and that could destroy evidence.

"That will be something that fire investigator will confirm in his report," he said.

Scene examinations at the two origin points have been completed and fire investigators are now piecing together the evidence.

He said it was possible the cause of either fire could come down to two or three different factors, and that's all investigators could prove.

"At that stage we use the words 'most likely to be caused by'. Ideally we would like to have just one cause, but that might not be possible" he said.

The time-line of the investigation was hard to judge, he said, with a huge fire area to look at.

"We need to consider all possibilities and there is quite a lot of information to digest. That takes time, and we need to let run its course," he said.

Detective Inspector Greg Murton said once the fire cause is determined, police can then decide if a criminal investigation will proceed.

"We have tracked down all the witnesses who saw the start of the fire and spoken with them. Until the fire service tells us how they started we are in a bit of a holding pattern," he said.

He said he could not comment on what evidence witnesses had provided until the fire investigation is complete.