Two Belgian tourists had planned to enter the river at Gentle Annie, but one lost his kayak before getting into the water, Detective Gary Hyndman, of Cromwell, said.
Motorists were concerned when they saw the kayak drifting without an occupant and contacted the police.
One of the Belgian kayakers went in the river to retrieve the empty kayak and was helped by two Mad Dog River Boarding guides, who swam across the river to make sure he was safe, and helped retrieve the lost kayak.
Mad Dog operations manager Brad Johnston said both kayakers were looking at the Citroen rapid, but one of them did not park his kayak high enough out of the water and it floated down the river.
"That's when the other kayaker said he would chase after it. They decided they would meet back at Roaring Meg."
The partner of the kayaker who chased the kayak asked Mr Johnston if he had see him.
"I asked the partner if they knew about the stretch of water. They didn't know it was a grade five rapid below Citroen, called Retrospect."
Mr Johnston said "shock" was the reaction from the woman.
She hoped her partner would stop and assess Retrospect before continuing after the kayak.
Concern for the kayaker's safety rose as time dragged on without hearing from him.
A 15-minute stretch of water was taking him 30 to 40 minutes to travel, Mr Johnston said.
"Talking to the police officer, there had to have been something going on and that's when we went up to the last point where he was last seen and jumped in the water to find him.
"Johno [Head] and I swam down to above Retrospect and then got out and walked down the river. Halfway down Retrospect we saw him walking on the opposite side, river-left.
"I whistled on my whistle to get his attention and I gave him the `are you OK' hand signal and he signalled he was."
Mr Johnston and Mr Head swam 8m across the fast-flowing river to the tourist.
"He was surprised that someone had raised the alarm, as in his mind he thought everything was fine.
" But when someone takes an hour to do a 15-minute stretch of water, that's a cause for concern."
While Mr Head updated police, Mr Johnston and the tourist kayaked together with the wayward kayak to above the natural bridge, the put-in point before the Roaring Meg stretch of water.
The tourist then rejoined his group.
"He was happy and very apologetic for taking so long, but really happy with the assistance we gave," Mr Johnston said.
"Knowing the river you're about to go down is key."
