The Cranbourne man, 37, came off his bike along the Henty Highway between Mockinya and Brimpaen about 3pm.

Senior Constable Shane Dignan said Horsham Highway Patrol was investigating the cause of the crash.

"The man was riding with a group of eight to 10 bikers and was out the front a bit," he said.

"The biker riding behind him could only remember seeing a cloud of dust.

"The rider ran off the shoulder of the road."

Investigators do not believe the man was speeding.

"We aren't ruling out the possibility of a mechanical failure on the bike," Sen Const Dignan said.

The speed limit on the stretch of road where the rider crashed was 100 kilometres an hour.

Ambulance Victoria spokesman John Mullen said paramedics took the man straight to Horsham aerodrome.