Courtney Carthy and a few friends took a float down the Yarra just before Christmas, in $15 inflatable boats from Kmart.

Mr Carthy had so much fun he decided to organise another inflatable paddle - so he put an event on Facebook for late March. When 500 said they were coming he remembers laughing to himself.

Courtney Carthy thought it would be fun to float down the Yarra with a few friends. Now, he's got 15,000 of them. Credit:Meredith O'Shea

He's not laughing any more.

"We're just at the tipping point to 15,000 people saying they're coming," says Mr Carthy, who grew up not far from the river, in Hawthorn and Surrey Hills.