A man has died while trying to save his son who had slipped off a cliff on a hike in Western Australia's Karijini National Park.

The father, 31, and his son, 7, were on a family holiday, hiking near Fortescue Falls in the Dales Gorge late yesterday when the accident occurred.

Acting Inspector Joe Watts told the media that the man had died and the son was critically injured before emergency help arrived.

A park ranger called for medical help when he saw the father and son lying unconscious. The ranger said the two must have fallen 8 and 14 metres.

"It's believed the man was fatally injured after he attempted to rescue his son after he'd tripped and fallen down an embankment near Fortescue Falls," said Senior Constable Naomi Smith.

The boy was taken to hospital in nearby town of Tom Price before being flown to Perth with his mother and two younger siblings, according to ABC News.

The Department of Environment and Conservation says Dales Gorge, including Fortescue Falls, Fern Pool, Three Ways Lookout, Circular Pool and the Dales day use area, are closed until further notice.