Major Collision Investigation Unit detectives are appealing for witnesses after a motorbike rider came off his motorbike in Knoxfield on Thursday 14 December.
Emergency services were called to the scene at Ferntree Gully Road about 7.20am.
It is understood a 65-year-old Ferntree Gully man, believed to be in his 60s, was transported by road to The Alfred Hospital in a serious condition with an upper body injury.
The exact circumstances around the incident are unclear.
It is believed that a large white SUV was travelling in the vicinity of the rider before the accident.
Investigators are urging the driver of that vehicle to come forward, as they believe they can assist with their enquiries.
Anyone who witnessed the incident, has dashcam/CCTV footage or information is urged to contact Crime Stoppers on 1800 333 000 or make a report online at crimestoppersvic.com,au.
