On a 10-day trip to New Zealand for a wedding, they were all "absolutely delighted" to be joined by Mr Key, who is also New Zealand's Tourism Minister, on the Shotover Canyon Ride.
Shotover Jet, now owned and operated by Ngai Tahu tourism, has been in operation since 1970, soon after the invention of the jet unit by New Zealander Bill Hamilton.
It began with wooden boats that carried five passengers and the company is now working on developing its fifth generation carbon-fibre jet-boats.
A damp Mr Key, who was in Queenstown as part of the Trenz international tourism trade event, said the ride was "sensational" and he commended the company on its lasting achievement.
"It's testimony to what is a fantastic ride and you think about the fact that it's been going for 40 years ... it's a fantastic part of the world, it's a great experience, it's fun and what a great way to show off New Zealand to Australians who screamed at me the whole way through it."
Despite his suit almost getting an unintentional wash - "I normally dry-clean them" - he recommended the experience to the rest of the world.
The official three-millionth passengers, husband and wife Nigel and Kristy Cox, of Melbourne, and Sara Anza and partner Adam Jambanis, of Perth, were ecstatic at the surprise.
Mrs Cox, who admitted to screaming during the ride, said it was one of the best things she had "ever experienced" in her life and when they were pulled out of the line thought they were "on Candid Camera".
"When we woke up this morning we had no idea that we would be meeting a prime minister. We were like `this isn't happening'."
Afterwards they were presented with commemorative jackets and beanies and treated to a scenic helicopter ride.
