Tory Ganderton was aged three years and nine months when he died 30 kilometres into the 150km trip between Shoalhaven Hospital and Sydney Children's Hospital, Randwick, in the early hours of September 10, 2014.

Admitted with a respiratory infection, he rapidly deteriorated in the back of the ambulance and could not be resuscitated despite CPR and intubation.

The inquest heard Tory's spinal muscular atrophy led to difficulty swallowing and coughing, and bouts of rapid weight loss.

Tory's loss of 11.5kg in the three months to September 2014 and his consequent malnourished state directly contributed to his respiratory failure, NSW coroner Derek Lee said on Wednesday.

The cheeky and optimistic boy - who loved his family, the Wiggles and his pet pig Peppa - left behind his parents Kayla Dowd and George Ganderton and two younger siblings Mikaylah and Billy-George.

"It's heartbreaking to know how suddenly Tory was taken from his family, and how much he is, and will continue to be, enormously missed every day," Mr Lee said.

Tory's deteriorating respiratory condition stabilised in Shoalhaven Hospital on the afternoon of September 9, but doctors were concerned Tory was on a downward trend and wouldn't receive adequate specialist care overnight if he became acutely unwell.

No helicopter or fixed-wing aircraft was available at the time.

"It was necessary for Tory to be transferred from Nowra to Sydney early in the morning of 10 September," the coroner concluded.

The pediatric retrieval team on board avoided sedating Tory out of concern the inherently fragile boy would lose respiratory drive and become "ventilator-dependent".

Mr Lee noted the "confronting challenge" the team faced and said the evidence established options involving intubation and sedation carried "too great a risk to Tory".

But the coroner said he couldn't be certain Tory's parents were told the exact reason the boy had to be urgently transferred and the limitations of caring for him in the ambulance.

"This ultimately had the regrettable and highly traumatic consequence of Kayla having to confront these issues in the midst of an obviously already distressing situation whilst resuscitation was being performed on Tory by the roadside," the coroner said.

Changes had since been made to ensure parents of children like Tory are offered the option to help design a resuscitation plan before acute care is needed.

Australian Associated Press