A couple killed in a fire along with their eight-year-old son had taken the batteries out of their fire alarms because they stopped working when their flat was too cold.

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Tej, 49, and Tika Kafle, 42, and their son Prem were killed in 2015 when an electric plastic kettle they bought two weeks earlier started a fire in their flat in Waimate, South Canterbury.

Mr Kafle arrived in New Zealand from Nepal in 2007 and the rest of the family, including his three daughters, joined him six years later.

They lived in various places before settling in Waimate where they ran the Everest Indian Restaurant on Queen Street.

The family lived on the flat above the restaurant and had been planning to move because it was too cold.

On 5 August 2015 Mr and Mrs Kafle and Prem had been in one room while the three sisters, who managed to escape through a window, were in another.

The alarm was raised when two volunteer firefighters saw smoke coming from the street, and the first crew arrived on the scene at 7.45am.

There was intense black smoke and limited visibility inside the flat and one firefighter at the scene said he could see very little because the room was smoke-logged and they had to crawl around.

Specialist fire investigator Kevin Collins said it appeared Mrs Kafle and Prem were awake when the fire started because they were both dressed. Prem was still in his school uniform.

It appeared Mrs Kafle took Prem down the hallway through the thick smoke, expecting he would be able to escape down the stairs.

"[She then] instinctively turned around and attempted to go back for her daughters and husband," said Mr Collins.

"She had only just got into the hall as far as the lounge door when she got caught in the fire flashover."

Prem's body was later found hunched up in the foetal position in the corner of the room, surrounded by clothing and blankets.

Mr Kafle's body was found next to the wall covered in blankets.

Coroner Marcus Elliot found they died because of carbon monoxide poisoning and smoke and fume inhalation.

It is not known why the kettle, bought from The Warehouse, ignited. More than 66,000 units of the particular model were sold in New Zealand and that was the first report of a fire being caused by the kettle that WorkSafe Energy Safety was aware of.

Mr Elliot said although there was no evidence that the type of kettle was inherently faulty, The Warehouse withdrew the item for sale out of respect to the family in March the next year.

The coroner also noted that the family's home did not have working smoke alarms because they had taken the batteries out.

Mr Collins found this was because when the temperature got too cold, the alarms would buzz.

Mr Elliot noted comments from Fire and Emergency which said it was important for landlords to ensure properties were well-insulated so that smoke alarms work well.

He also said to reduce the risk of fire from any small appliances, they should be switched off at the wall after being used.