A woman has been evacuated off the top of the Sydney Harbour Bridge, after suffering heat exhaustion while completing the bridge climb.

The woman, aged in her fifties, was climbing the Harbour Bridge on Saturday afternoon when she suffered heat exhaustion.

Six ambulance crews attended and treated the woman on top of the Harbour Bridge.

Emergency services helped the woman down stairs from the top of the bridge down to the road below, where she was placed in an ambulance, according to a spokesman for NSW Ambulance.