Police investigating the cause of the Port Hills fire have confirmed one of the fires is now officially considered suspicious.

The Port Hills fires started as two separate blazes on February 13 this year, with the two fires merging into one by February 15.

The cause of the devastating fires has since been investigated, and now the Marley's Hill fire is officially considered suspicious.

Detective Inspector Greg Murton said the development meant police would step up their work to find the cause and origin of the fire.

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"Police are seeking any new information from the public and would like to hear from anyone who has information in regard to the fires if they haven't yet spoken with us.

"Our investigation is well advanced, evidence gathering and inquiries at the scene have been completed and a range of witnesses have been spoken to.

"These fires had a devastating impact on the Port Hills and wider community and we encourage anyone who has information that may help find the person responsible to come forward.

"Someone in the community will have information that is crucial to this investigation and we encourage you to make contact with us."

Fire and Emergency office of the chief executive director Leigh Deuchars asked that locals stay patient, as investigators couldn't say much now that the investigation was a criminal matter.

"We understand this may be frustrating for residents who are anxious to know more information about the fires.

"The investigation into the second Cause and Origin Fire Investigation Report is still ongoing and expected to take at least a couple of months.

"The immediate focus for Fire and Emergency New Zealand in the wake of the fires was on putting them out, ensuring the safety of the community and limiting the damage to property as much as possible.

"When a fire is suspicious, it becomes a criminal investigation managed by the Police.

"These investigations are taking some time to complete given the scale, damage and amount of information, some being very technical, that needs to be considered."

Anyone with information is asked to called Christchurch Police on 03 363 7400, or anonymously through CrimeStoppers on 0800 555 111.

The Port Hills fire burned through 2000ha of forestry and 11 homes, and required a large firefighting operation over several days to put it out.

Former SAS soldier and experienced pilot Steve Askin died trying to fight the flames, with a report showing cables from a monsoon bucket caused a fatal accident.

The cost of fighting it reached $7.9 million, including $2.1m for aircrafts costs alone.