A coroner has been unable to say if the death of a Hastings woman set on fire was suicide or a killing after key witnesses refused to talk because of gang connections.

Coroner CJ Devonport has ruled the death of 42-year-old Amelia Whatarau was caused by burns she received after being doused with petrol and set on fire at her Whakatu home on Christmas Day, 2012.

Whatarau died in hospital five days later, with burns across more than half her body and face. But with a lack of evidence, the inquiry has been unable to conclude who caused the blaze.

Police told the coroner several key witnesses had refused to make statements because of the Mongrel Mob connections of Whatarau's partner of 20 years, with whom she lived.

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There had been more than 25 cases of reported violence between the pair since 1994, there was domestic protection order in place and the couple had "very animated" arguments, including on the day of Whatarau's burning, the coroner said.

When emergency services arrived, Whatarau was found near scorching on the ground outside, burned from the waist up and smelling strongly of petrol while her partner - who eventually went into shock from his own burns - was hosing her down.

Since the incident, the partner and two other women present at the home had refused to be interviewed or make formal statements, the coroner said.

Police said they had tried to question Whatarau's partner, including at a police station with his lawyer present, but that he only said he would not be making a statement before leaving.

"With key witnesses refusing to make statements ... at this stage there is insufficient evidence to determine whether Ms Whatarau died as a result of suicide ... or as the result of actions of another party."

There was no law requiring witnesses to give evidence to an inquest, the coroner said.

"I am not confident an inquest would advance my inquiry any further."

- NZN