John Turnbull was working near the bridge and said the first he knew of the incident was turning around to see his workmate Shaun Pirere dripping wet and talking to police.
Mr Turnbull said Mr Pirere told him he'd seen the woman jump, hold on to the bridge pylon for a bit, then let go.
"He said she floated for about 20 seconds, then went under."
That was when Mr Pirere took action, swimming to the middle of the river and hauling the woman back to shore.
"She would have been a goner if it hadn't been for Shaun," Mr Turnbull said.
The woman was taken by police to Whanganui Hospital where she was checked by medical staff, and found to be uninjured.