Witnesses to the death of a Whakatu woman, who died as a result of burns on Christmas Day, have closed ranks and will not tell police what happened.
Amelia Koriana Whatarau, 42, received burns to the upper half of her body and later died from the injuries in Auckland's Middlemore Hospital.
Her 49-year-old partner was treated for burns to his hands and arms.
"At this time the people who were at the address when Amelia suffered her burns have chosen not to formally speak to police about this event," Detective Senior Sergeant Dave de Lange said.
"This is very frustrating from an investigative point of view and hard on Amelia's extended family as we cannot progress this matter to a resolution until we are sure what has happened. Obviously we will keep investigating this matter until this is resolved."
Forensic evidence, located at the scene and from expert witnesses, will be reviewed.
Emergency services were called to the family's rear section in Buckingham St at 7.10pm.
on Christmas Day.
Neighbours reported hearing "screams that somebody was on fire" after a large cloud of black smoke was seen rising from the back yard where the couple lived with their daughters.
"We heard her yelling and screaming all day and he was yelling and screaming all day," a neighbour said at the time.
"The kids kept coming out and then we saw the smoke ... it wasn't very nice at all. They have quite a few domestics in there but this is the worst they have ever had."
A neighbour who rang emergency services was taken to hospital with a heart complaint that a relative said was made worse by the incident.
"It's a horrible thing, especially on Christmas Day, for something to happen like that," she said.
Sensible Sentencing Trust spokesman Garth McVicar said the inability of police to interview reluctant witnesses or suspects was "appalling".
"The law should be changed so the police have the opportunity to speak with witnesses or suspects at the earliest opportunities," he said.
"At present you are getting the accused, or witnesses who do not want to finger the accused, colluding to take the police off the scent."
Police can only detain people for questioning if they are under arrest. Suspects under arrest still had the right to silence.
The threshold for arrest is expected to increase when the Criminal Procedure (Reform and Modernisation) bill becomes law in July. The right to silence was questioned by Starship hospital's director of child protection, Dr Patrick Kelly, at the 2011 inquest into the deaths of babies Chris and Cru Kahui from traumatic brain injuries.
Mr McVicar said the Kahui case was a "classic example".
"Each one of the people at the house had a lawyer present who at that stage wouldn't let the police interview them.
"So that power needs to be reinstated for the police, we believe."
The police's job had become more difficult because New Zealand had gone "on a politically correct journey".
"From out point of view that's disgusting, particularly when you see guilty parties getting away with crimes," Mr McVicar said.