Brimbank Sexual Offences and Child Abuse Investigation Team (SOCIT) detectives are investigating two incidents where a man was seen inappropriately touching himself in Sunshine West last month.
Both incidents occurred in the vicinity of Kororoit Creek Trail between Wright Street and Derby Road.
During the first instance, two 17-year-old girls were out walking when they saw a man lying naked in the grass masturbating on Friday 11 September about 3.20pm.
The man followed them as they walked away towards Derby Road where they lost sight of him.
The second incident occurred on Monday 21 September, where a male cyclist noticed a man following behind a female jogger and masturbating about 1.40pm.
The bike rider was about to alert the 31-year-old jogger as to what was occurring behind her and the offender ran off along Wright Street.
The man is perceived to be Asian in appearance, about 165cm tall with black hair cut in a short military style.
He was wearing black shorts, a blue top and grey jacket.
Police believe it is likely the man has targeted other women or teenage girls in the area and urge those people to contact Brimbank SOCIT.
Any witnesses or anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au