A three-year-old girl has fallen through a fly screen and out a first-floor window of a house on Sydney's northern beaches.

CareFlight said the girl hit an awning before falling to the ground from her Elanora Heights home just before 1pm (AEDT) on Wednesday.

The girl suffered head injuries, a CareFlight doctor said.

She was placed in an induced coma and flown to the Children's Hospital at Westmead where she remains in a serious but stable condition.