Kane Heal, 22, was in Christchurch Hospital's intensive care unit today after the accident in North Canterbury on Saturday morning.
Police said on Saturday that Mr Heal suffered serious facial injuries when he slid along the road.
Senior Constable Chris Hughey of Hanmer Springs police told the Nelson Mail Mr Heal had his trousers down and was mooning other vehicles from the car's roof when he fell off at a speed of at least 80km/h.
He said Mr Heal required facial surgery after sliding 20m face-first along State Highway 7 about 10km north of the Hanmer Springs turnoff.
Mr Hughey said Mr Heal had climbed on top of a Nissan car driven by his girlfriend as they travelled from Nelson to Christchurch.