The motorcyclist was struck by the car on State Highway 1 about 400m north of the Shag Point turn off about 4.30pm, a police spokeswoman said.
A St John spokesman said the man was taken to hospital by rescue helicopter in a serious condition.
One of the occupants of the car was treated at the scene for minor injuries, he said.
It is unclear how many people were in the car.
The police spokeswoman said the motorcyclist was breathing when police arrived at the scene but was unconscious.
"They have a broken pelvis and two broken wrists,'' she said.
Early indications suggest the motorcyclist was hit when the car crossed the centre line into his path to enter a layby on a straight stretch of road, Senior Sergeant Brian Benn said.
A Dunedin Hospital spokesman tonight said it was too early to comment on the man's condition.
"He remains in the emergency department. He's still being assessed at the moment,'' he said about 8.15pm.
The road was blocked immediately following the crash, but soon opened to one lane of traffic. It has since been reopened.
