At breakneck speeds of up to 85km/h, in river water often just a few centimetres deep, they tore through the towering Shotover canyons just outside Queenstown.
Lucky for the royal couple, they had just steeled themselves for the daredevil ride by enjoying a wine tasting event at Amisfield Winery down the road.
Suggestions Kate might be pregnant were dispelled with news she had indulged in a tipple.
And they needed all of their Dutch-courage to keep their eyes open as they flashed past rocky outcrops, skimmed around jutting and ragged boulders, and sped through the narrow canyons.
The driver - with all the skill of a Formula One driver or fighter jet pilot - suddenly took a hand off the steering wheel and made a helicopter signal above his head. It's the sign to brace yourself, Kate, you're about to do a 360-degree spin at high speed.
While the drivers are highly skilled, it only takes one small mistake or a mechanical failure, and it's curtains: sheer, unforgiving rockface curtains.
William, sporting a black New Zealand cap complete with silver fern, and a dressed-down Kate in skinny jeans, were plainly thrilled by the 25-minute trip, and afterwards skipped up the gangplank, buzzing.
Photographs of the royal pair will no doubt be added to Shotover Jet's 'Wall of Fame'.
They will join other celebrity thrillseekers, including Brooke Shields, Jonah Lomu, John Key, George Lucas, and Knights of the Realm, Peter Jackson, Peter Blake and Ian Botham.
No doubt, William and Kate will be pride of place.
