Casey Crime Investigation Unit detectives are investigating four separate incidents, which took place between 5 August and 29 August.
Police believe the same teenager was involved on each occasion.
Armed with a knife, he approached women on the walking track along Gunns Road Reserve and demanded their possessions.
The first incident occurred about 5.45pm on 5 August, when a 41-year-old Hallam woman was jogging east along the track.
She passed a teenager who was riding a bike in the opposition direction.
The teen came up behind the woman and produced a knife before fleeing with her phone.
The second incident took place on the same track on 12 August about 2.30pm.
Two Hallam women, aged 55 and 29, were walking up the hill towards the Power Road overpass when they were approached from behind by a teenager.
He demanded the women hand over their possessions, however the older woman used her umbrella to fend off the male before he ran to hide in some nearby bushes.
The third incident occurred about 2pm on 26 August.
The victim, a 21-year old Endeavour Hills woman, was walking along the track when she saw a teenager riding a push bike in the opposite direction.
A short distance later, the woman saw the same teen approaching her from behind.
He produced a knife and demanded she hand over her possessions.
She handed over her phone and he fled west along the track.
The fourth incident occurred on 29 August about 4.30pm.
A 55-year-old Doveton woman was running along Kevin Adlard Reserve towards Gunns Road Reserve when she saw a teenager with a push bike on the side of the track.
She continued running when she was grabbed on the left shoulder.
The woman turned and saw the same teen, who again demanded the woman hand over her possessions.
She fled and the offender rode off empty-handed.
Investigators previously released a composite image of a teen who they believe can assist with their enquiries.
He is perceived as African in appearance and aged about 15 years old with a skinny build and short dark hair.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au.
