Naked jogger thought it would 'boost his immune system'

A Sydney man who was spotted jogging in the nude at a public park has pleaded guilty to obscene exposure.

Stojan Karlusic was found in Sydney Olympic Park on a brisk June evening, wearing only a plastic bag to cover his genitals and his running shoes.

And why? He said he heard exercising naked in the middle of winter, would boost his immune system.

On the day he was found, the 42-year-old said he was testing out the theory.

He began by taking off his clothing, but leaving his underwear on, before deciding he needed to push himself.

Karlusic admitted to police it wasn't the first time he had done so, although it was the first time he had been caught.

Court documents obtained by 7 News show when park rangers approached and asked him what he was doing, Karlusic responded: "I'm sorry. It was me exercising naked - just a silly prank."

He is due to be sentenced later this month, and could face a fine or up to six months' jail.