Police investigating an Auckland house fire that claimed four lives earlier chased an anonymous tip alleging it had been started by a person with a grudge, a Coroner's report has revealed.

Bhamini Theiventhiran, 39, her 5-year-old son Bareth Kailesh and 66-year-old mother Umadhevi Theiventhiran all died in the tragic blaze in east Auckland's Flat Bush in December 2016.

Theiventhiran's 47-year-old husband Kaileshan Thanabalasingham was severely injured and died in hospital about one month later.

Mourners attend the funeral of Bhamini Theiventhiran, Umadhevi Theiventhira and Bareth Kaileshan in January 2017. Photo / NZ Herald
Mourners attend the funeral of Bhamini Theiventhiran, Umadhevi Theiventhira and Bareth Kaileshan in January 2017. Photo / NZ Herald

A coroner's report into Theiventhiran's death today found she died from burns and smoke inhalation.

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The blaze was New Zealand's worst house fire since the 1970s as Theiventhiran, her son and mother-in-law were caught sheltering in the ensuite bathroom of an upstairs bedroom.

But fire investigators say the family would likely have survived if they had known to close internal doors in the house - a measure that would have given firefighters time to reach them.

"Tragically, our investigation shows that if they had closed the doors, the intense heat and toxic smoke from the fire probably wouldn't have reached them," Fire and Emergency's fire investigation national manager Peter Wilding said.

Coroner Sarn Herdson said investigators had determined the fatal blaze likely began as a smouldering fire in lounge room furniture or a rug on the ground floor of the two-storey house.

The possibility it was caused by an ember from a cigarette could not be proven but could also not be ruled out, she said.

The fire was not deemed deliberate.

However, police had - earlier in the investigation - chased a lead in which they were given an anonymous note "alleging a local person may have been responsible for starting the fire".

The note said the person had been seen outside Theiventhiran's home the day after the fire and that they had earlier been unsuccessful in buying the house and so held a grudge against the family.

"Police investigated the allegation but found no evidence to support this specific allegation, or to support a finding that the fire was suspicious," Herdson said.

Bhamini Theiventhiran's husband Khailesh Thanabalasingham, who was secretary for the New Zealand Refugee Council. Photo / Supplied
Bhamini Theiventhiran's husband Khailesh Thanabalasingham, who was secretary for the New Zealand Refugee Council. Photo / Supplied

All the family members who slept upstairs ultimately died as a result of the fire.

However, Theiventhiran's father Theiventhiran Vinasathamey and her daughter Krishah Mantra Kaileshan had been sleeping on the ground floor and escaped.

The family had earlier eaten dinner together, watched television and gone to bed around 10pm.

Vinasathamey said he got up to use the ground floor bathroom about 1.30am but did not notice or smell anything to concern him.

However, two hours later, between 3am-and-3.20am on December 22, 2016, he woke and felt the heat of the fire and heard glass breaking.

When he opened his bedroom door, there was thick smoke and flames.

Waking his granddaughter - who was asleep in the same room - he then called out to family members sleeping upstairs before running out the front door on to the street.

Theiventhiran's husband Thanabalasingham - who had been sleeping upstairs - also managed to escape the house, despite being badly hurt, by climbing through a window and down a drain pipe on the house's wall.

He was found collapsed on a nearby driveway and taken to Middlemore Hospital where he remained for about one month before dying.

Fire and Emergency's Wilding said quickly escaping from a building that's on fire is usually the safest option but that isn't always possible.

"If you're trapped inside, you can increase your chances of survival by shutting all doors between you and the fire, then placing bedding or clothing along the bottom of the door to keep the smoke out," he said.

"This buys precious time for firefighters to arrive."

He said fire investigators also found a number of other safety issues in Theiventhiran's house.

The home had one smoke alarm, but would have been better served by a collection of "interconnected smoke alarms".

"This increases the chances of people being alerted to a fire, giving them more time to escape," he said.

Coroner Herdson's report recommended authorities look at whether current building codes could be amended to require smoke alarms and water sprinklers to be better installed in multi-level housing.

Investigators also learned neighbours had heard sounds of windows breaking but didn't immediately call emergency services, Wilding said.

"We'd rather respond to a 111 call and find we weren't needed because it was a false alarm, than not be notified early enough. As communities, we need to look out for each other," he said.

The Coroner's report also recommended authorities consider reducing the flammability of materials used in common household furniture.

Large, padded furniture containing polyurethane foam are the single biggest contributor to the speed that fires develop in New Zealand homes, Wilding said.

"In a fire, they quickly produce intense heat and release highly poisonous gases, meaning there is less time for people to escape," he said.