The man, believed aged 20 and from Kanahooka, is understood to have been riding with friends when he came unstuck on Jamberoo Road shortly before 2pm on Monday.

He suffered serious head and leg injuries and was bleeding internally. He stopped breathing for a period, before he was resuscitated on the roadside by emergency services.

A highway patrol crew raced extra donated blood to the crash site, near the intersection of Swamp Road, after treating paramedics used up those supplies on hand.

NSW Ambulance Illawarra chief inspector Terry Morrow said police officers performed first aid on the man before paramedics arrived.

"The police were here first and it was a pretty daunting sight for them," he said.

"The patient wasn't in a very good condition. He was unconscious at the time and he needed resuscitation.

"He's come around a corner, he's lost control and he's gone straight under a 4WD."

The man departed the crash site about 3.30pm in a helicopter bound for St George Hospital.

"At the moment he's in a critical condition," Insp Morrow said.

"We've now given him some blood products ... we'd already used three vials of whole blood for him because he was bleeding internally. We asked highway patrol to go to Shellharbour Hospital to pick up some blood products for us so that we could continue resuscitating him."

As of 8pm on Monday the rider was still in a critical condition at St George Hospital.

Passing motorists including Jamberoo's Lyntton Tonta were among the first to come to the injured man's aid. Mr Tonta said he found the rider with a bleeding head wound, and called triple-zero.

Jamberoo Road is closed to traffic in both directions between Kiama and Jamberoo.

Light vehicles are being diverted onto Fountaindale Road; heavy vehicles will use Albion Park.