Police believe a car and a motorcyclist were speeding side-by-side before the rider crashed and died in Melbourne's outer south-east overnight.
Acting Sergeant Brett Trewin said witnesses reported the motorcyclist and a red sedan turning south from Sladen Street, Cranbourne, onto the South Gippsland Highway about 11:25pm
"Witnesses have seen the red sedan and the motorcycle travelling side by side, by which time the red sedan and the motorcycle have accelerated heavily," Acting Sergeant Trewin said.
"Witnesses have then come round the bend to find the male motorcycle rider has come off his motorcycle."
Emergency services treated the man at the scene but were unable to revive him.
Major Collision Investigation Unit detectives are investigating the incident, and are appealing for the driver of the car to come forward.
The dead man is yet to be identified.
The death brings the state's road toll to 70 for this year, compared to 87 at the same point last year.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000.
