Two children have been killed in a ferocious fire that swept through their Hawkesbury home on Monday morning, hours before they were due to relocate to Queensland with their parents and four siblings.

The children were reported missing by their family in the blaze at their two-storey house on Lieutenant Bowen Drive in Bowen Mountain about 2.10am. Their father and four siblings escaped the property with minor injuries, while their mother had already left for Queensland.

The fire tore through the home at 2am on Monday, and was out of control for two hours.© Nine News

Responders were forced to wait for hours before they could search the property, as the fire burnt out of control. The bodies were discovered on separate storeys of the house in searches that began after the fire was contained at 4am.

More than 50 firefighters faced a “very overwhelming situation” when they arrived to find flames tearing through the home, collapsing the roof and other internal structures. Emergency responders made the “incredibly tough” safety decision to retreat after an early assessment suggested the walls could also collapse.

The ages of the six children in the family ranged from three to 16, according to Hawkesbury Police Area Command Superintendent Nadine Roberts.

The father and four children who escaped are being treated for smoke inhalation and minor burns at Nepean Hospital, while the mother is returning to Sydney.

“It’s an incredibly confronting situation not only for the community and families but the first responders who had to attend,” said Roberts, who did not confirm the ages of the children killed.

Police have established a crime scene at the home, and are working to formally identify the bodies. They do not believe the fire is suspicious.

RFS Superintendent Jonathon Hill, one of the first firefighters on the scene, said “words can’t really describe” the difficulty of the morning.

“It’s a situation that you really can’t comprehend until you’re in it,” Hill said. “We want to help the community, we do our best. And again, there’s a human side of this that you can’t always explain.”

Drones and dogs were deployed to assist with rescue efforts at the property, which continued to smoulder for hours after the fire was extinguished.

NSW Fire and Rescue Assistant Commissioner Andrew Faunce said an investigation would determine the origin of the blaze and circumstances leading to the fire, as well as the number of smoke alarms and other safety devices in the property.

“Residential homes now can be very quick to go from low-level fires to significantly involved,” Faunce said referring to the severe stage of the fire.

“We’re not aware of when the fire started, but when firefighters arrived it was significantly involved and that led to the decision not to do an active fire attack.”

Lieutenant Bowen Drive is a long residential street with homes on its western side directly bordering the Blue Mountains. The fire has caused no damage to neighbouring homes.