A woman who fell from a moving campervan on the Waikato Expressway was trying to shut a door when wind blew it open, sending her tumbling on to the road.

She remained in Waikato Hospital's Intensive Care Unit in a critical condition this afternoon, a hospital spokeswoman said.

The woman fell on Thursday trying to shut a door.

Waikato road policing manager Inspector Marcus Lynam said it appeared the door of the campervan wasn't closed properly.

"The woman has gone to close it and the wind caught it before she went out the door.

"Speed was not a factor in the incident - you don't have to be going fast for something like that to happen," he said.

"While it's still in the early stages of the investigation, this incident appears to be a freak accident and neither speed nor alcohol were believed to be factors."

The woman and her partner were travelling south on State Highway 1 in their campervan when the woman went to shut a door that wouldn't close properly.

The 59-year-old's husband was driving at the time. He was understandably upset, Waikato police Senior Sergeant Rupert Friend said yesterday.

The couple were New Zealanders travelling home "further down the line", he said, unsure where.

Friend reminded motorists to pull over before attempting to check on any noises that may have come from their vehicle.

"I think the biggest thing is if you think there's anything wrong with your vehicle make sure you stop at the location before having a look at it or attempting to fix it."

The serious crash unit attended the scene but he didn't believe there was anything wrong with the vehicle.

Swerving skid marks were all that was left at the scene, at the exit to the Avantidrome, after the campervan was taken away.