Northern Grampians Highway Patrol police are appealing for a driver who witnessed a serious crash in Glenthompson last month to come forward and assist investigators.
A motorcyclist was travelling northeast along Maroona-Glenthompson Road on a yellow BMW motorcycle just after 11am on Sunday 18 April.
A police vehicle was travelling in the opposite direction and observed the bike skid, lose control and crash.
Officers stopped and assisted the rider, a 61-year-old New South Wales man, who was airlifted to hospital with serious injuries.
Investigators are appealing witnesses, particularly the driver of a small silver sedan which was seen travelling behind the motorcycle.
The exact circumstances surrounding the collision are yet to be determined.
As per standard protocol, detectives from Professional Standards Command and Major Collision Investigation Unit will oversee the investigation.
Anyone with information that could assist police with their enquiries is urged to contact Crime Stoppers on 18000 333 000 or submit a confidential crime report at www.crimestoppersvic.com.au