One part fun run, three parts fluoro mayhem, the Swisse Color Run brought about 20,000 people to Sydney's Centennial Park on Sunday in the name of simply running amok.

While the idea of getting up early on a Sunday morning to go running may not rate high on everyone's scale of enjoyment, organisers said the latest version of the race added to the event's claim to being the "happiest five kilometres on the planet".

Race director Luke Hannan said about 7500 kilograms of coloured, food-grade corn starch was thrown during the race, which does not formally declare a winner.

"We celebrate the last runner as we celebrate the first," Mr Hannan said, adding that the threat of rain had not prevented runners from showing up.

"We are so lucky that the gods and the sun were shining on us … quite literally. This is the largest event we have had in Australia so far."

The Color Run, a for-profit business, is held regularly across cities in Australia and throughout the world.

Meanwhile, in a very different foot race on Sunday, more than 850 climbed 1504 steps to the Observation Deck at the Sydney Tower Eye.

Mark Bourne and Suzy Walsham won the men's and women's races, helping raise more than $150,000 for autism charity Giant Steps.