A woman who was knocked out when she was thrown from a horse in the NSW Blue Mountains has been flown to hospital by rescue helicopter.

Ambulance paramedics called the CareFlight chopper shortly after midday (AEDT) on Saturday after the 41-year-old was bucked off while riding with a friend in the Kanimbla Valley, near Hartley, west of Blackheath.

The CareFlight doctor said the 41-year-old woman was left unconscious for about five minutes.

She was flown to Nepean Hospital, where was reported to be in a stable condition.