Fire investigators do not know what caused the December Auckland house fire that this morning claimed its fourth victim.

Refugee Council of New Zealand secretary Kaileshan "Kailesh" Thanabalasingham, 47, escaped the blaze in Flat Bush on December 22 but was severely burned. He died in hospital this morning.

His 11-year-old daughter and 69-year-old father-in-law also escaped the fire, which claimed claimed the lives of Thanabalasingham's 5-year-old son Bareth Kailesh, 39-year-old wife Bhamini Theiventhiran and 66-year-old mother-in-law Umadhevi Theiventhiran.

Counties Manukau fire risk managment officer Phil Faidley said despite exstensive investigations it was impossible to work out the cause of the fatal fire.

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"We have ascertained the fire's origin but unfortunately because we are unable to be conclusive, the cause of the fire is undetermined."

He said the fatal fire started in the home's groundfloor lounge and dining area and would have been smouldering before bursting into flame.

"The indications are the fire had been a low, smouldering fire that might have been burning for 90 minutes," he said.

There was at least one working smoke alarm in the house that survivors of the fire reported hearing as they escaped.

Faidley said today's death, which added to the fire's already grim toll, was tragic.

He had understood that despite grave injuries suffered by Thanabalasingham he was slowly improving.

"I really feel for the family and I feel for the community and I feel for our guys as well," he said.

Family spokeswoman, barrister Deborah Manning, told the New Zealand Herald Thanabalasingham died at Middlemore Hospital just before 7am.

"The family are obviously devastated and asked for privacy. It's a devastating shock."

Family were with Thanabalasingham when he died, she said.

She was unsure if Thanabalasingham's daughter had been told of her father's death.

"As you can all imagine this is just very new information."

Refugees as Survivors New Zealand today was shocked by Thanabalasingham's death as he was believed to be rallying in recent days.

"Refugees as Survivors New Zealand is deeply saddened to learn of the death of our dear friend and colleague Kailesh Thanabalasingham. In the last few days he had shown signs of improvement which makes his loss all the more devastating," said the group's chief executive Ann Hood.

She said their friend's life encapsulated the refugee spirit, arriving here with nothing but creating a safe place for his family to grow up as well as working tirelessly for his community.

"He touched the lives of so many people with his warm and sunny nature and his compassion and determination to make a difference. New Zealand is a richer place as a result of his efforts to foster a humane and inclusive society where all people are respected and valued."

Thanabalasingham was heavily sedated after the fire and was not able to attend the funeral of his family members at the Manukau Memorial Gardens this month.

It is understood he was not aware of the deaths of his son, wife and mother-in-law.

A Givealittle page set up for the family by the Refugee Council and has so far raised $80,000.