CAPE TOWN (AFP) - A South African man on Saturday successfully flew across the sea from Nelson Mandela's apartheid island prison using helium-filled giant party balloons.

The six-kilometre crossing, to raise funds for a children's hospital named after the country's former president, was the first stunt of its kind from the historical site.

Matt Silver-Vallance, 37, took around an hour to float across the Atlantic Ocean from Robben Island while harnessed to a mass of multi-coloured balloons in grey, drizzly conditions with low visibility.

Making his way wearing a wetsuit, he floated a few metres above the sea, with controls for flight including bags weighted with water and an air gun and make-shift spear to pop balloons.

"Wow, that was crazy," he said, saying he felt "unbelievable" after landing in a rubber duck around 300-400m from shore once the balloons were released.

"Don't try this at home," he quipped.

With no test run ahead of lift-off, a total of 160 balloons were inflated on the island early Saturday morning, with several popping ahead of departure.

Silver-Vallance popped around 35 more balloons during the trip to manage his equilibrium. A hard ground landing was ruled out as too risky.

The daring mission aimed to raise 10 million rand (S$1.4 million) for the Nelson Mandela Children's Hospital which will be built in Johannesburg.

"We're trying to raise as much money (as possible) for the Nelson Mandela Children's Hospital and we really see this project as a catalyst," Silver-Vallance said ahead of take-off.

The hospital will be part of Mandela's legacy and the balloon run was a "small thing" to try to remind people of everything the 94-year-old anti-apartheid icon had done, he said.

"The risks that I'm taking are tiny compared to the risks that he took," he said, adding he did not consider himself a dare-devil.