A 50th birthday party went horribly wrong at a house in Medowie on Saturday night when five people were injured from a backyard heater explosion.
Two people remain in a critical condition with severe burns and another three have serious burns from the incident, which police said occurred when a diesel heater was being re-fuelled.
One of those in a critical condition is a serving member of the Royal Australian Air Force.
"Defence was saddened to learn of an explosion in Medowie on Saturday and can confirm that an Air Force member was injured," a Defence spokesperson told the Newcastle Herald.
"None of the others injured were Australian Defence Force (ADF) members.
"Due to the seriousness of the member's condition, no further details will be provided at this time."
Emergency services were called to the house in Carnoustie Way shortly after 8pm on Saturday night.
A 50-year-old Medowie man and a 42-year-old Corlette woman who sustained severe burns were stabilised at the scene by NSW Ambulance paramedics and two helicopter medical crews.
They were airlifted in separate helicopters to Royal North Shore Hospital in a critical condition.
Two Medowie men, aged 39 and 54, and a Raymond Terrace man, 40, were treated at the scene for serious burns to their bodies. All three were transported by road to John Hunter Hospital.
Police established a crime scene at the house on Saturday night with witnesses assisting their inquiries.
Multiple Fire and Rescue NSW officials returned to the property on Sunday morning as investigations continued.
A post-it note with a hand-written message advising of the birthday party was still stuck on one neighbour's front door on Sunday.
Other neighbours said one of the residents of the house where the incident took place had distributed similar messages as a courtesy.
Proving the quiet nature of the residential area, many neighbours said that they were not even aware the incident had taken place.
Others said they had heard commotion and then approaching emergency service vehicles.
"We heard a bang," one neighbour said.
"We didn't think anything of it, there was a party on.
"We've only been here 18 months, this is the quietest street ever."
The two people airlifted to Royal North Shore Hospital remain in a critical condition, a spokesman for the Northern Sydney Local Health District said on Sunday night.
The 39-year-old Medowie man is in a stable condition at John Hunter Hospital while the two others have been released, Hunter New England Health said.