Police said the man was standing in the underpass at O'Halloran Circuit about 6pm, when he grabbed the female cyclist who was riding past.

He pulled her off her bike and placed his genitals against her mid-section before letting her go.

The man is described as caucasian, thin and between 170 to 180cm tall with light brown hair.

He was wearing a black bicycle helmet, black T-shirt and black shorts at the time.

Police ask anyone who might have witnessed the incident to contact Crime Stoppers on 1800 333 000 or at www.act.crimestoppers.com.au.

Information can be provided anonymously.