"I was in Spain at Port Aventura on shambhala ride and saw the person a few rows ahead drop their IPhone X," he wrote in the video description.
"Long story short, I caught it. This roller coaster is moving at over 130kms... was once Europe's tallest and fastest coaster.. only recently beaten."
Once they were off the rollercoaster, Kempf handed the phone back to its owner, who was looking for the device on the ground.
"He couldn't believe it, he gave me a big hug," Kempf told Stuff.
"The funniest reaction was when we had got off the rollercoaster and you go down to look at the photos and videos they take and half the people there seemed to be watching our screen and then everyone started cheering and clapping when it happened."
The owner of the phone bought the video as a thank you to the Kiwi.
People on YouTube love the Timaru man's incredible dexterity.
"In all my years on the internet this is probably the coolest s**t i've ever seen," one person commented.
"I didn't know Jesus himself resurrected," someone else said.