Two young siblings have been taken to hospital after plunging 10 metres from the third floor of an apartment block in the city's west.

Emergency services were called to an apartment complex at South Terrace, Bankstown, at 8.50am on Sunday, responding to initial reports that two children, aged three and five, had fallen out a window.  

Police said the five-year-old sustained life-threatening injuries after she hit the concrete pavement near the entrance to the apartment block.  

Paramedics treated the two children at the scene before they were taken to The Children's Hospital at Westmead by road ambulance for further treatment.

The girl's younger brother escaped with minor injuries and is now in a stable condition.

Police from Bankstown Local Area Command are investigating.