People left in charge of vulnerable infants were advised to "exercise their duties responsibly", southern region coroner David Crerar said in his formal written findings, released on Thursday, on the death of an Invercargill infant in 2010.
Joseph James Batchelor-Smith (9 months) was put to bed by his caregiver, Honey Samson, on September 25.
When she went to get him up 17-18 hours later, he was dead.
Pathologist Noelyn Hung, a senior lecturer at the University of Otago, found his death was attributed to an acute episode of septicaemia with an associated fever, exacerbated by the heat in his home.
It was likely the bacteria in his bloodstream had come through an eczema sore.
Analysis of his urine indicated his muscles had begun to break down, which would have occurred before his death. That was consistent with his death being associated with hyperthermia, "also in context with the layers of clothing from which he probably could not extricate himself and the ambient air temperature".
Prof Barry Taylor, of the University of Otago, said in the two weeks before his death Joseph had eczema, which caused breaks in the skin and increased the risk of infection.
"On the night he was put down to sleep, baby Joseph was dressed in a stretch-and-grow, a singlet and a nappy, then wrapped in a blanket, possibly with more than one layer, with another blanket over him and then a double layer of duvet."
The mounted heat pump in the lounge had run all night, and ambulance officers noted it was set at 25degC, with no windows open.
"Prof Taylor identified a risk factor in that Honey Samson and [her partner] Quentin Pascoe slept in a room some distance away from the lounge with probably two doors shut between the lounge and their bedroom.
"[Prof Taylor] advised the inquest hearing ... 'should he [Joseph] have tried to alert caregivers to his plight, they were not within earshot and able to respond to his needs.
There are multiple factors contributing to his death of which only one would be the high environmental temperature'."
Mr Crerar said it was "of concern" Ms Samson did not check Joseph for up to 18 hours.
"Whilst I have no evidence which could satisfy me that if baby Joseph had been checked on earlier in the night or in the morning by his caregiver, his distress, and probable high temperature, would have been noted and action ought to have been taken ..."
Joseph's mother, Georgina Daisley, left him with Ms Samson on the afternoon of September 25, intending to return the next day.
Mr Crerar said Joseph was fed that night by Ms Samson and Mr Pascoe, his nappy was changed and he was put to bed between 7.30pm and 8.30pm.
Mr Pascoe got up at 11am the next day and went into the lounge, where Joseph slept in his cot.
"He could see baby Joseph lying on his side, in the cot, and facing the wall. He appeared to be asleep. [He] did not check on the baby."
More than three hours later, about 2.34pm, Ms Samson went to get Joseph up to take him with her to a tangi.
"She discovered that he was deceased."
Ambulance officer James Stronach described the room as "bloody warm; rather hot actually" and said it was apparent Joseph had been dead for some time.
Mr Crerar said he was not able to state unequivocally Joseph's death was caused by overheating, in that the ambient temperature of the sleeping accommodation, in conjunction with his clothing and bedding, was inappropriate.
"I draw to public attention the fact that high temperatures in living, and particularly in sleeping, accommodation is a risk factor for the premature deaths of vulnerable infants. Low temperatures are of course also a risk factor.
"Families of babies ought to take advice on, and learn how to care for, their children in all circumstances."
He recommended a copy of the findings be forwarded to the Child and Youth Mortality Review Committee so the circumstances of Joseph's death could be publicised and such deaths did not recur.
