Tourists among four or five others on the raft suffered "a couple of bumps, but nothing of any consequence."
The spill was on the last major rapid of a tough grade-five rafting section of the river, on which the lodge's website cautions would-be guests: "You may fall out - in fact 5 per cent of people have an unplanned exit from the raft."
Mr Megaw said the lodge was upfront about the challenges and risks.
"We can't have customers coming here who think they have zero chance of an involuntary exit from the raft," he said.
But customers received three safety briefings before setting off, and guiding standards had improved considerably in recent years, making injuries requiring hospital treatment rare.
"In years gone by we might have had one a year - now it's one every two or three years," he said.
"We raft about 4000 people a year and have very few injuries, but unfortunately he [the guide] landed on a rock."