The men were working on the new $22million Kawarau Falls bridge when the incident happened.
Nick Whetter and colleagues Kevin Speight, Jamie Edgar and Paul Wallace were drilling on two platforms shortly after 3pm, when they noticed a jet-boat head up the Kawarau River into Lake Wakatipu.
Moments later Mr Whetter saw "two bodies floating back down the river''.
He said the outcome could have been very different if they had not been there. He did not think the couple "would have made it'' otherwise.
"There was tool kits, petrol cans and other bits floating down with them. I thought crikey ... we had life rings on the platform so we grabbed those and rang 111.''
The couple, described as in their late 60s and from out of town, were thrown from the boat when it flipped, sending them back down the Kawarau River.
Mr Whetter and his workmates threw a buoy to the pair but were unable to reach them.
"We couldn't get it out fast enough and they floated past us. The guy was trying to call help. I don't think the woman had enough energy to do that.''
The couple floated towards trees, on the Kelvin Heights side, and Mr Speight scrambled on to a tree that had fallen into the river. He managed to grab hold of the pair and haul them out and up on to the bank.
It was a bit of an exercise, he said.
"The woman was very cold. They did have life jackets on, which will have saved them. The lady said she went under a couple of times and swallowed some water.''
The rescue took just over five minutes.
