A man is in hospital with serious head injuries after falling 15 metres down a waterfall in the Blue Mountains.

Police said the man, 29, was hiking with his family at Wentworth Falls in the mountains west of Sydney when he climbed over a safety fence, slipped and fell down the waterfall, about 2.30pm on Saturday (AEDT).

Police said in a statement the man landed in a shallow pool.

He was winched onto an ambulance helicopter and flown to Westmead Hospital.

A police spokeswoman said on Monday the man was being treated for serious head injuries.

He is in a serious, but not life-threatening condition 29-year old man has survived a 15-metre plunge down a waterfall in the Blue Mountains.

The accident happened on the same day abseiler Darren Bull was killed when he plummeted down a cliff at Malaita Point in the mountains.