The house where a fire killed a ten-year-old boy and two other people in Sydney's south-west didn't have smoke alarms, authorities say.

Five people, including two firefighters, have been hospitalised after the early morning blaze.

At about 5.40am, ten fire trucks responded to an emergency at a townhouse on Rottnest Avenue, Hinchinbrook.

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Two women, who are yet to be formally identified but are believed to be in their 40s and 60s, died at the scene.

The 10-year-old boy was taken to the Children's Hospital at Westmead in a critical condition but he later died from his injuries.

Three other people who were rescued from the house remain in hospital: a man in his 40s, who is in a critical condition, and two women aged in their 30s and 40s, both of whom are stable.

Fire and Rescue NSW Super Intendant Luke Unsworth said two crew members were also injured at the scene.

"One firefighter received an electric shock and one firefighter fell through a weakened flood and they've injured their back," he said.

Both firefighters were taken to hospital in serious conditions.

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Fire and Rescue NSW crews extinguished the fire.

The cause of the blaze is not yet known, and an investigation is underway and a crime scene has been established.