A young man who groped a woman while she was out running ended up being tackled to the ground when it turned out his victim was a competitive kickboxer and triathlete.

Renee Ker had been out running in Cairns, Australia, early in the morning when a male, believed to be aged between 15 and 17, rode up behind her on his bike and grabbed her late last week.

Ms Ker, who is a former competitive kickboxer and now represents New Zealand in triathlons, said she "saw red" and instinctively started to chase him.

"He rode up behind me and grabbed my backside and slipped his hands between my legs," Ms Ker told the Daily Mail.

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"I took off after him because I'm an ex-kickboxer. I launched into him and knocked him off his bike.

"I was about to punch him before I realised he was quite young."

Ms Ker said she screamed for help because it was the one day she didn't take her phone but her attacker broke loose and fled before anyone arrived.

He left behind his bike, helmet and hat.

The former Ironman athlete said her attacker "freaked out" because he wasn't expecting it.

"He absolutely picked the wrong woman... or maybe the right woman. I'm glad it was me and not someone else. I don't play the victim very well,' she said.

Off to Cozumel Mexico this year to represent NZ at the worlds ITU Triathlon grand final again.. Boom! Time to get busy!

A post shared by Renee Ker TriMotivate Coach (@trimotivate) on

Police are still investigating the incident and are trying to track down the man who indecently assaulted Ms Ker on Thursday, March 31 on Lake Morris Road.

The man is described as being aged between 15 and 17 years of age, slim build, dark complexion, short dark hair and around 160 to 170cm tall.