UPDATE 1.49Pm: A set of swerving skid marks is all that remains on the Waikato Expressway of the terrifying moment a woman fell from a moving campervan.

The woman is in a serious condition and has been airlifted to Waikato Hospital after the incident occurred on State Highway 1 at Tamahere.

A police spokeswoman said the woman fell about 11am. The left-hand southbound lane of the road was briefly closed near the Cambridge velodrome exit after the accident.

She was a passenger in the vehicle travelling south on State Highway One near Cambridge when the incident occurred, police said.

"Initial reports are that a side door was opened to check on a noise, and then the person has fallen out," a police spokeswoman said.

"She's received serious injuries."

The Serious Crash Unit are investigating.