Update: The 29-year-old Augusta man was catapulted from the jet ski after hitting a shallow sand bar in the water about 300m from the mouth of the Blackwood River.
He landed on the sand bar, which was covered by only about 20cm of water, about 10.40am. The sandbar was about 1.5km from the boat ramp at the Colour Patch cafe in Augusta.
His brother, who was on another jet ski, and a man fishing nearby helped him and raised the alarm.
An Augusta Volunteer Marine and Rescue Group boat, with a St John’s volunteer ambulance officer on board, rescued the man and took him to shore, where he was transferred by ambulance to Augusta Airport.
The chopper landed about 11.35pm and is expected to arrive at Royal Perth Hospital about an hour after take off.
A St John Ambulance spokesman said the man received serious spinal injuries in the accident.
It is understood the man was conscious.
Locals say sandbars in the area can shift quickly.