Two bodies were found on Monday morning after a blaze that broke out in the early hours at a home in the small town of Bowen Mountain, in the foothills of the NSW Blue Mountains.
They are yet to be formally identified but police believe they are two children who were unaccounted for after their father and four siblings escaped the inferno and a roof collapse.
NSW Police Superintendent Nadine Roberts said the rest of the family managed to escape the blaze, which happened on the morning they were due to move to Queensland.
The mother of the children - who range in age from three to 16 - had been on her way to their new home in Queensland at the time of the fire, she said.
"Our thoughts and prayers are with the families, the siblings and with the broader community," Supt Roberts said.
Neighbours tried to help with initial efforts to fight the fire but couldn't get inside the property due to the intensity of the blaze, authorities said.
The fire was not currently being treated as suspicious, Supt Roberts added.
About 60 firefighters from Fire and Rescue and the Rural Fire Service were needed to bring the blaze under control about 3.30am, but many were forced to withdraw before the roof caved in.
NSW Ambulance confirmed four people were taken to Nepean Hospital for smoke inhalation and minor injuries.
The four included a man aged in his 30s, a teenage male and two primary school-aged boys.
Officers have established a crime scene at the home.
An investigation into the cause of the fire and formal identification of the two bodies had commenced, NSW Police said in an earlier statement.
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