A large family about to embark on a major relocation have been caught up in a house fire tragedy that claimed the lives of two young children.

Two bodies have been found after a blaze broke out early on Monday at a home in the small town of Bowen Mountain, on the foothills of the NSW Blue Mountains.

They are yet to be formally identified but police believe they are two children who were unaccounted. Their father and four siblings escaped the inferno, which caused a roof to collapse.

One body was found on the top floor of the house and the other discovered downstairs, NSW Police Superintendent Natalie Roberts said.

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 aftermath of Monday’s deadly blaze, in which two children are believed to have died. Photo: ABC News

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,” 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 being treated as suspicious, Roberts said.

Rural Fire Service Superintendent Jonathon Hill said that when firefighters arrived, the blaze had already burned through much of the roof cavity of the house and caused some structural damage.

About 60 firefighters from Fire and Rescue and the Rural Fire Service brought the fire under control about 3.30am. They had earlier been forced to withdraw before the roof caved in.

“When firefighters arrived, we had a fully developed fire … with structural collapse already occurring,” Hill said.

“It was deemed too unsafe for firefighters to make entry, to conduct any kind of rescue or search.”

Fire and Rescue NSW Assistant Commissioner Andrew Faunce said other family members had already evacuated from the house by the time emergency services arrived.

Family members told firefighters that there were still people inside the burning structure, he said.

Faunce urged people to ensure they had winter fire safety plans, and check that their smoke alarms were in working order.

Roberts confirmed a father and four children who escaped the blaze were taken to Nepean Hospital for smoke inhalation and minor burns.

NSW Police said earlier a crime scene had been established at the home, with an investigation under way into the cause of the fire.

-AAP