Greater Dandenong Crime Investigation Unit is appealing for information following an incident in Keysborough last month.
Investigators have been told the victim was riding his distinctive red Harley Davidson motorcycle along Hutton Road about 7pm on 21 March.
Two males driving a black Holden Commodore Ute indicated for him to pull over in relation to an issue with his motorbike.
The rider pulled over and one of the male’s in the car approached him and struck him to the head with an unknown weapon.
The offender jumped on the motorcycle and rode off on Hutton Road toward Springvale Road.
The male that stole the motorbike is perceived to be of Pacific Islander/Maori appearance, approximately 183cm tall and was wearing a black hooded jumper.
Investigators have released a facial composite of a male they believe may be able to assist with their enquiries.
An image of the stolen motorbike has also been released which has registration YBEAR.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or make a confidential on-line report at www.crimestoppers.com.au