The man's mobility scooter was just visible beneath the water line.
"He wasn't responding to any of the attempts. Somebody came with an emergency kit and tried to put a breathing mask on him but he wasn't responding to that either."
Paramedics arrived and worked on him for over an hour but they pronounced him dead at the scene.
Singh spoke to a witness who said he saw the man driving the mobility scooter on the walkway beside the river when he somehow veered down a bank near the bridge and into the water.
Police said they were in the process of notifying next of kin and establishing the circumstances of the death.
The matter will be referred to the coroner.