Two children have died in a horror house fire in the Hawkesbury region of NSW, after the blaze caused the roof of the two-storey property to collapse.

Their father and four other children managed to escape the blaze at the home in Bowen Mountain, about 77 kilometres north-west of Sydney.

The family had planned to relocate to Queensland today, with the mother of the children already on her way there when the flames erupted about 2am.

Four children managed to escape the blaze. (Nine)

She is now on her way back to the Hawkesbury to be with her family.

Firefighters were on the scene within “seven to eight minutes” of the alarm being raised, yet arrived to a “very, very developed” fire, Rural Fire Service (RFS) Superintendent Jonathon Hill said.

“When firefighters arrived, we had a fully developed fire at the time, with the roof collapsing,” he said.

“It was deemed unsafe to enter the house at that point in time.”

The blaze developed so quickly that it was unsafe for firefighters to enter the home once they arrived at the scene. (Nine)
The surviving family members were taken to Nepean Hospital. (Nine)

Crews were forced to temporarily retreat and take defensive action.

It took two hours for 12 fire trucks and about 50 firefighters to bring the blaze under control, with emergency crews beginning their search of the home about 4am.

They found one of the victims downstairs and one upstairs.

The surviving family members are in Nepean Hospital being treated for smoke inhalation and minor burns.

Those who died in the fire have yet to be formally identified, but police say the children in the household range in age from three to 16.

A crime scene has been established and the cause of the fire is under investigation.

The house was destroyed in the fire. (Nine)

Police Superintendent Nadine Roberts said police “are certainly not treating it as suspicious” at this stage.

It was unclear whether the property was fitted with working smoke alarms.

NSW RFS Commissioner Trent Curtin said the crews had no choice but to initially take defensive action.

“Firefighters want to get in there with their hoselines and getting in and locating people who might be missing, but an early assessment of the building recognised that parts of the walls were looking to collapse, and parts of the structure had collapsed,” he said.

“It would have been too dangerous to go inside the home while it was collapsing.”

Firefighters were forced to take defensive action after arriving, as parts of the house had collapsed. (Nine)
NSW RFS Commissioner Trent Curtin said firefighters had to initially retreat due to the dangerous nature of the situation. (Nine)

“Some parts of the home are hot and smouldering, and many parts of the home have collapsed,” Curtin said.

Curtin praised the actions of responding crews to ensure the fierce fire didn’t spread to other homes, and he said all firefighters would be offered support.

“[It’s] very tough for our firefighters… all of those firefighters are impacted by the circumstances,” he said.

Investigations have commenced into how the fire started.