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Emergency services were called to the scene of the accident, between Lindsay Rd and the Southern Motorway, about 6.30pm.
It was unclear what led to the man scaling the closed bridge, but police last night were not treating the matter as criminal.
Station Officer Simon Smith, of Lookout Point, said he believed the man had fallen between 8m and 10m.
"We helped stabilise him with ambulance [personnel] and get him out with a Stokes basket," he said.
The man was conscious but in a great deal of pain.
A St John spokesman said two ambulances were sent to assist the man after he fell shortly before 6.30pm.
The man had suffered serious injuries, however, St John could not provide details of the nature of those. He was taken to Dunedin Hospital’s emergency department.
A police spokeswoman said it appeared it was an accident.