A four-wheel drive has fallen off a barge heading to Fraser Island and sank.

No-one was inside at the time, but a group of backpackers on their way to the island for New Year's Eve lost their mobile phones, bank cards, passports and other valuables when the rented vehicle rolled out between Inskip Point and Fraser Island, north of Brisbane, about 10:00am.

Backpacker Chloe Swift, who was on the barge as part of the same backpacker tagalong group, said it was a "really sad and scary start" to her Fraser Island trip.

"There were a lot of girls obviously crying. Their phones and everything are in the car that sunk," she said.

The barge company is investigating the incident. ( Audience submitted: Chloe Swift )

"The guys took it quite well but a lot of the girls were crying and a lot of people haven't rung their families because they are so upset right now.

"It was really horrible, a lot of the people were shaken up.

"The people who have lost their stuff have to go back to the hostel and borrow stuff they have lost."

Brisbane woman Katrina Lawrence, who was on the barge with her family, said their vehicle was parked next to the one that rolled off.

"We were behind our car and my husband called out that the car next to us was rolling. It was rolling back to the bridge which was still down," Ms Lawrence said.

"A young guy tried to grab it by the bull bar but he didn't have much luck.

"It skidded along for a bit, he let it go, and it proceeded to just roll over the back and into the water where it stayed up for about 30 seconds before it sunk down.

"It was really slow. It was like it was happening in slow motion but there was nothing anyone could do to stop it."

A spokesman for Manta Ray Fraser Island Barges said they were investigating the incident and declined further comment.