The cause of the fatal fire in Flat Bush which killed three members of the same family is officially undetermined following an investigation.

Fire investigator Phil Faidley said while the investigation team had a theory about what had caused the fire, they were not able to conclusively determine how the fire started.

Bhamini Theiventhiran, her son Bareth Kailesh and her mother Umadhevi Theiventhiran died shortly before Christmas.

They were farewelled in a funeral service at the Manukau Gardens last Thursday.

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The trio died when a fire ripped through their Flat Bush home on December 22, leaving Bhamini's husband Kaileshan Thanabalasingham with burns to 40 per cent of his body and in a medically-induced coma.

The couple's 11-year-old daughter and Bhamini's father were the only two to make it out of the fire unscathed.

"We're recording an undetermined finding," Faidley said.

This was the category of cause used when an investigation was unable to conclusively determine the exact cause of a fire.

"We do believe the fire was accidental. It was a very tragic accident."

Faidley said he was unable to discuss what they believed the cause was as they had not been able to conclusively prove it.

He said this was not uncommon in fire investigations, where a hypothesis was created and then tested using available information.

A formal investigation report was currently being put together.

It was possible that if new information came to hand the case could be reopened.

"If any additional information comes to hand then we may be in a position to be a bit more specific about what's caused the fire," Faidley said.

"We are calling this a very tragic accident and our process now says 'let's look for learnings we can take away from this'."

Police are working with the family to inform them of the investigation's findings, Faidley said.

Police have referred the three deaths to the coroner.

A police spokeswoman said as the fire service was the lead agency and the fire was not deemed suspicious, police had no further comment.