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