Ten months ago Deajay Parkinson-Batt died in Palmerston North Hospital when life support was switched off after he was rushed to ICU in critical condition.

But after an exhaustive investigation involving a team of highly trained detectives and pathology experts, police say they are stumped by the 2-year-old's mysterious death and have no idea how he died.

The toddler - whose favourite toys were cars and trucks - was in the custody of Oranga Tamariki (OT) at the time of his death.

And though a doctor told family Deajay had suffered a severe brain injury, the police investigation has now been scaled back due to lack of evidence and no one has been charged.

Deajay's grieving birth parents, Talia Parkinson, 23, and Dion Batt, 24, say the pain of losing their only son is still unbearable and they are demanding answers about how he lost his life.

"They said they didn't know what happened," Batt told the Herald.

"It took over half a year to get a report. The last 10 months were all for nothing with no answers."

Deajay was admitted to hospital on June 1. He died early the next morning surrounded by family.

Dion Batt and Talia Parkinson are demanding answers about how their son Deajay Parkinson-Batt died while in state care. Photo / Mark Mitchell

In a distressing legal bungle, police later arrived at the funeral home demanding Deajay's body, which they planned to return to the OT caregiver.

After the funeral director explained the birth parents had legal ownership of the body - even if Deajay had been in state care - the officers realised their mistake and left empty handed.

Police later acknowledged the error but stopped short of apologising to the family.

Parkinson told the Herald the insensitive gaffe compounded the couple's suffering at a time when they were planning their child's tangi.

A post-mortem examination was carried out on Deajay's body before it was released to the funeral home.

The results were "inconclusive" despite extensive tests being sent to experts for review.

The coroner received the final forensic pathologist's report from police last month and will now begin his investigation.

A video of Deajay taken on the last day Parkinson saw her son alive shows her "baby boy" dressed in a Batman costume laughing in his car seat.

"We took him for icecream and fed the ducks at the square," Parkinson told the Herald through tears.

The agony of losing her son was made worse by not knowing what caused his death.

Police told the parents Deajay had a swollen brain and dehydration, but there was no evidence of any injuries.

Parkinson and Batt said the carer who was looking after Deajay at the time of his death had not contacted them, nor had anyone from OT passed on their condolences.

"Our son's gone and they don't even want to talk to us."

In a statement, a police spokeswoman said the death remained a mystery.

"As a result of the investigation, which included a detailed and thorough post-mortem examination and subsequent testing, police have been unable to establish the cause of death and are now liaising with the coroner.

"Police have conducted a thorough investigation with an unwavering commitment to finding out what happened to Deajay. The investigation timeframe is a consequence of this work and the required testing."

A large team was involved in the investigation but had been "scaled back" with the passing of time.

"Police acknowledge how immensely difficult it is for Deajay's whanau and friends to not yet have the answers they deserve.

"Our team has worked hard with that express goal and we now hope the coronial process can provide some sense of closure."

Police had received full co-operation from all those who loved and cared for Deajay throughout the investigation, the spokeswoman said.

Officers met with Deajay's birth parents last month to update them on the post-mortem results.

Oranga Tamariki, deputy chief executive Services for Children and Families South, Alison McDonald, said the death of any child was heartbreaking.

"We acknowledge the pain and suffering of all who knew and loved Deajay."

McDonald would not comment on what communications the agency had had with Deajay's birth family, or whether Deajay's caregiver was still providing care to children in OT custody, citing privacy.

"The matter is now before the coroner and we are committed to learning what we can from their findings."

A coronial spokesman said Coroner Peter Ryan was now reviewing this "tragic case".

It was too soon to say whether an inquest would be held and hard to predict how long the inquiry would take. The coroner must consider evidence from a wide range of sources. The family would be supported and kept informed during this process.

Parkinson last saw her son alive two months before his death.

The night he was admitted to ICU she received a text message from a family member telling her Deajay was fighting for his life and that she should come to the hospital.

She was initially turned away at reception before a family member let her up to ICU.

"I just had this feeling, like I already knew," Parkinson said.

"He was in bed on life support. No one would say anything. I told everyone to just pray.

"After a couple of minutes the doctor said, 'He has a severe brain injury', then, 'He's not there any more'.

"We just said our last words and he smiled at both of us. His eyes went all watery and I know he was crying."