It is believed a red Honda motorcycle was travelling south along Myers Creek Road when it collided with a green Toyota Hilux travelling in the opposite direction about 10.40am on Saturday 6 October.
The rider, a 62-year-old Eltham man, died at the scene.
The male driver of the vehicle, a 30-year-old Werribee man is assisting police with their enquiries.
Four other occupants of the car were not injured.
Police have been told a red vehicle was seen by a cyclist driving behind the green Toyota prior to the collision and left before police arrived.
Investigators are keen to speak to the driver of that red vehicle and any other witnesses.
No one has been charged and the investigation remains ongoing.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au