"I came out of the gym and went for a run after training, after my second lap around the block I noticed a car parked on the street,' she said.
"On the third lap I noticed it had moved back, well out of residential view.
"I thought 'that's odd'. Then I noticed the passenger doors were open and that made me a little suspicious."
Trying to convince herself she was just being paranoid, the woman nevertheless veered off onto the grass to avoid running directly past the man's car.
"As I did I noticed the man, with no pants on and no underpants on, holding himself in his hand. It was really shocking."
Police said they were able to locate the man after the woman called 111.
"The person who reported the incident told police he was sitting in his car, naked from the waist down and watching her, he then drove in her direction," a police spokeswoman told the Herald.
"The man and his vehicle were no longer in the area but police took a statement from the woman who could recall the car's registration.
"Police were then able to locate the vehicle and its owner. A man is now assisting police with their inquiries."
The woman said she looked younger than she was and worried the man had thought he was targeting a teenage girl.
"People do that sort of things and they target younger girls," she said.
"What really worries me is if he was targeting the kids and young girls in the area, who would be too scared or embarrassed to report it."