The "tragic case" of two young boys who died in a bathroom over 10 hours while their mother lay on her bed only metres away has highlighted the need for ongoing awareness of postnatal depression, the WA Coroner has found.
Handing down an open finding on Tuesday, State Coroner Alastair Hope said it was "unfortunate" that so many features of this "unusual case" would remain a mystery.
Malachi Stevens, 10 months old, and his brother Lochlan, 2, died after their mother Miranda Hebble left them in a shower with the water running and then either passed out or fell asleep about 3.30pm on November 7 2008.
She woke to water at her feet nearly half a day later and found the lifeless bodies of her sons, the inquests heard last month.
"I passed out and my babies are in the shower and the door closed and they are dead," Ms Hebbard told a triple-0 operator on November 8 2008 at 1.30am.
"They're not breathing, they're dead, I ... passed out at 2 o'clock today and I only just woke up."
No cause of death could be established for either boy, although drowning was not ruled out for Malachi, who was found in the flooded shower recess.
Lochlan, however, was found on the bathroom floor, and examinations suggested he died some time after his brother.
"For two children to die within hours of each other in somewhat different circumstances appears to be a remarkable turn of events," Mr Hope said.
An examination of Malachi's body after his death also revealed he had been failing to thrive for months and had he been taken to a doctor he would likely have been admitted to hospital for medical examination.
Lochlan's post mortem examination revealed he had early stages of pneumonia, a sign of acute infection in the lung, a mild concussion and low blood sugar.
His death was consistent with hunger and exposure, while drowning could also not be ruled out as a contributing factor.
Following a week-long inquest, Mr Hope found there was no medical evidence to explain Ms Hebble being unconscious while the boys died, and that it was likely she was simply in an "extremely deep sleep".
But he also noted her older son Lochlan would have died over a "significant amount of time" and made noise in the hours before his death.
"Why she was not roused is not known," Mr Hope said.
He also described an incident some months earlier, in which Ms Hebble had left a then five-week-old Malachi in a hot car as "concerning" in light of the deaths.
"Clearly Malachi could have died when left alone in a hot motor vehicle in the car park of a shopping centre," he said.
The condition of the family's home after paramedics attended had suggested Ms Hebble was also struggling to cope with domestic tasks, Mr Hope found.
"The environment was an unhealthy one for two young children," he said.
"There is a significant body of evidence which indicated that prior to the deaths Ms Hebble was increasingly suffering a deterioration of her level of functioning.
"In addition, Ms Hebble was becoming increasingly socially isolated, and it appears likely that she had no direct contact with any other adult in the days before the deaths."
Mr Hope said even in the event Ms Hebble had passed out, the act of leaving two young children in a shower and walking away was in itself "a high-risk action".
"There are a considerable number of features of this case which are unexplained and may never be known," he said.
"This is a very unusual case.
"Clearly the children did not die from natural causes, but the circumstances in which they died have not been revealed by the evidence at the inquest."
Mr Hope said while it could not be ascertained whether Ms Hebble had been suffering from postnatal depression following the birth of her second child, there was significant evidence to suggest she was suffering from "impaired functioning".
He made no recommendations.
But he said the case had "highlighted the need for there to be ongoing community awareness of this issue and the importance of there being adequate support mechanisms in cases where the demands of coping with young children, often in relatively socially isolating circumstances, can set off an escalating cycle of distress for mothers and give rise to unacceptable threats to child safety".