"Everyone who saw the accident was in shock."
Witness Malinda Dunwoodie, 17, said the girl's safety harness was still down but it looked as if she slipped out of her seat.
"She was hanging by her legs off the ride, trying to block herself. She went flying through the air about 10-15 metres up in the air and I started sprinting towards her," she told News Corp Australia.
"I was shaking so much, she looked like a doll flying through the air.
"It was so shocking. It was like a horror movie."
Police were unable to confirm reports a second girl was injured on the ride.
The ride was a new addition for the Royal Adelaide show this year but it was included at shows in Sydney and Brisbane earlier this year without incident.
Its 12 carriages are at the end of arms that radiate from a central pivot, hurling patrons around and up and down at the same time.
Police and SafeWork SA are investigating the incident and will prepare a report for the coroner.
Royal Agricultural & Horticultural Society chief executive John Rothwell said all rides at the show were subject to daily checks and a national compliance audit.
The Airmaxx 360 ride would be shut down for the remaining two days of the show but other rides would continue to operate, he said.
"We've gone for years without problems," he told reporters.
"It's a rare occurrence. This is a tragedy and we have no idea what's caused it.
"As a parent, I can only imagine the grief the family must be feeling right now."
Police are seeking contact from anyone who went on the ride before the tragedy.
Adelaide woman Anna Burston told reporters her young son Beau went on the ride about an hour before the fatal incident and became scared when his carriage started spinning on its own.
There were height restrictions on the ride but no age or weight restrictions.
SafeWork SA executive director Bryan Russell said he was not aware of any complaints about the ride.
"All of the critical items have been regularly checked," he said.
In 2000, 37 people were injured when the Spin Dragon ride collapsed in sideshow alley at the annual show.
- AAP
