Neighbours reported hearing screams for help after smoke was seen coming from the two-storey home on Montana Way, Macquarie Fields, at 1.35am today.
Crews reported flames of about two metres coming out of all of the windows.
While almost everyone in the house was able to escape the fire, a teenage boy was unable to be found.
Once firefighters put the fire out, they went inside the home and found a body in an upstairs bedroom.
NSW Fire and Rescue Deputy Commissioner Jeremy Fewtrell said the size of the fire put “tremendous pressure” on crews.
“They were aware that there are people in danger inside the building and they were doing their best efforts to get to those people,” Mr Fewtrell said.
“It’s been a devastating day for the family (and) the person involved but also for our crews who’ve got that heartache to work through as well,” the spokesman said.
Police said they believe a female occupant and her five children were able to escape the home and a visitor, a 14-year-old boy, was not accounted for.
NSW Police Superintendent Julian Griffiths said it was too early to formally identify the body and police were still in the preliminary stages of the investigation.
Police said they were canvassing the street and a man is assisting with their enquiries.
Mr Griffiths described the incident as “a tragic situation where a young life has been lost”.
“I think it’s paramount that we as the police investigate the circumstances,” he said.
“We encourage the community to assist us in that regard by letting us know if they have information.”