The female victim was walking along a footpath just off Toomuc Valley Road in Pakenham about 6.30am on March 2 when she was attacked.
She tried to call for help during the assault, but the man threatened to kill her before releasing her when he was interrupted by a passing car.
The woman was going for a walk when she was approached by the unknown male on a bike who asked her for the time.
She pulled out her mobile phone to check the time when the attacker grabbed her by the neck and forced her to the ground.
After the car drove past, the man fled 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, aged 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 crossbar and a blue and white frame.
Meanwhile, a teenager has been charged during the investigation but he is not being treated as a suspect for the sexual assault.
A 16-year-old Officer boy was charged with theft and deception and has been bailed to appear at a children’s court at a later date.