Dandenong Sexual Offences & Child Abuse Investigation Team are investigating a sexual assault in Pakenham last month.
A 60-year-old woman was walking along a footpath just off Toomuc Valley Road in Pakenham about 6.30am on 2 March.
She was approached by an unknown male on a bicycle who asked her for the time.
The woman pulled out her mobile phone to check the time when the offender grabbed her by the neck and forced her to the ground.
He sat on her, demanding the password for the phone whilst sexually assaulting her as she was pinned down by her neck.
The victim tried to call for help but the man threatened to kill her, releasing her when he was interrupted by a passing car and fleeing on foot with her phone and credit card.
The woman was not physically injured and managed to flag down a passer-by for help.
The male is perceived to be Caucasian, 18-30-years-old, 160-170cm tall and was wearing a black mask, black cap and black jacket with the hood pulled up.
The bicycle he was riding had a blue cross bar and a blue/white frame.
Investigators have released images and CCTV of a male they believe may be able to assist with their enquiries.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au