Portland police Sergeant Ryan Nelson said the rider was heading north towards Heywood just after 8pm when the accident happened on a slight right-hand bend.

He said the rider and his motorcycle left the road and the man was thrown from the bike.

"Investigations will be continuing on Monday morning but there don't appear to be any extenuating circumstances," Sergeant Nelson said.

"At this stage, there's no indication another vehicle was involved.  

“Portland police are investigating, Warrnambool Highway Patrol unit officers attended on Sunday night and Warrnambool crime scene officers will examine the accident site on Monday morning," he said.

Police are appealing for anyone with information to contact Crime Stoppers on 1800 333 000.

EARLIER, Sunday: Police are investigating the circumstances of a fatal crash at Heathmere tonight. 

It appears a male motorcyclist was thrown from his motorbike on Princes Highway, near Portland, just after 8pm, a Victoria police spokeswoman said. 

Paramedics were called but the man, who is yet to be formally identified, died at the scene. 

Investigators are appealing for anyone with information to contact Crime Stoppers on 1800 333 000. 

The provisional road toll is at 168, compared to 161 at the same time last year.