Police have urged parents to be more vigilant after a five-year-old girl fractured her skull in a fall from a window at her family's home in Forster on the NSW mid north coast.

Police said the girl was playing on the window's sill with her six-year-old sister when she fell through the window screen onto a concrete slab about six metres below about 8.30pm (AEDT) on Monday.

Ambulance officers took her to Manning Base Hospital in Taree with a fractured skull, fractured ribs and bleeding on the brain.

She was later transferred to Newcastle's John Hunter Hospital, where she is reported to be in a stable condition.

The girl's fall has prompted police to urge anyone caring for children to be vigilant when children are near open windows or in high-rise accommodation.

Police said parents should ensure windows were secure, could not be opened more than 10cm by children or have a securely fitted window guard.

They said window fly-screens were not "child-proof", parents should not place furniture in front of windows, windows should be locked and closed if a child was left without adult supervision.