A 16-month-old baby who died at Whanganui Hospital is believed to have been accidentally left in a car on a hot summer's day.

Police said yesterday they were investigating the death of the baby boy in the grounds of Whanganui Hospital last Friday afternoon.

The mother had intended to drop the child at a nearby early learning centre that morning.

It was only later that the mother realised the toddler was in her car.

She did not usually take the child to the centre on Fridays. The father was unable to do so, as normal, on the day of the death because he was working, NZME reported.

The temperature reached 26 degrees Celsius that afternoon.

It's understood that when the mother rang the childcare centre, she was told the toddler hadn't been dropped off that day.

Police have not publicly confirmed the full circumstances of the death.

Police acting central district crime manager Detective Inspector David Kirby yesterday said an autopsy was carried out on the boy the next day.

Other medical tests were taking place and police were speaking with the baby's family and hospital staff.

Police were not looking for anyone else to talk to in relation to the death, Kirby said.

"Any death of a child is always tragic," he said.

"However, it is too early in the investigation to comment on what happened and what the outcome of the investigation will be."

A Whanganui DHB spokeswoman said there was a "sudden and tragic death within the Whanganui Hospital grounds" on Friday.

"Our thoughts are with the family," she said.

CARS REACH DANGEROUS HEAT IN MINUTES

Paediatric Society of New Zealand president Dr David Newman said it was hard to comprehend what a devastating incident this would be for the family.

Newman worked in South Australia in the 1990s and said he had seen similar cases happen there.

"During one summer there were three children who died in the back of one car, in plain sight of hundreds of people at a fairground, because no-one knew they were trapped in a car which reached 55 degrees [Celsius] in temperature, which no-one can survive in," he said.

It did not take an extremely hot day to heat the inside of a car to dangerous levels, he said.

When the outside temperature was just 21C, the temperature inside a car could reach 49C in just 45 minutes as the sun radiated through the windows, he said.

"An adult could not survive in that heat and children are far more susceptible to it. They overheat three to five times faster than an adult."

FORGOTTEN BABY SYNDROME

The phenomenon of normally attentive parents inadvertently leaving their children in cars was known in some parts of the world as Forgotten Baby Syndrome.

READ MORE: Forgotten Baby Syndrome: It can happen to anyone

University of South Florida molecular physiology professor David Diamond has told the Washington Post that stress - either sudden or chronic - could weaken the brain's higher-functioning centres and give people a type of "tunnel vision" where they forgot about their child.

"The quality of prior parental care seems to be irrelevant [in these cases]," he said.