The Secret Life of Us and Rush actor, who announced his retirement last year, took out the top prize at the annual Moomba Festival competition on Sunday before an excited crowd.

"I've wanted to do this since I was a kid," he said moments before his successful leap into the murky waters of the Yarra River.

Johnson, 38, donned a colourful costume in the event, but bailed on carrying his glider made of bras as he dove off the platform in a forward somersault.

Courageous competitors are judged each year on how far they travel across the river, but they came make up extra points by raising money for the charity of their choice.

Johnson dedicated his big win to his sister, Connie, and their cancer charity, Love Your Sister, which they launched after she was diagnosed with breast cancer when she was 33.

He raised more than $25,000 for the charity during his birdman leap.

Johnson, who starred as music industry icon Molly Meldrum in Molly last year, was up against 14 others who were jumping into the soggy sport.

His glider entry was called "Titty Titty Bang Bang".

Johnson announced his retirement from acting last year in order to focus on his charity work.

The first Birdman Rally was held in 1976, but the Moomba tradition took a six-year hiatus until it returned in 2004.