The family of a girl who nearly died from a box jellyfish sting say the nearby campers who poured vinegar over her saved her life.

Rachel Shardlow, 10, was tangled in the jellyfish tentacles while swimming with her 12-year-old brother Sam in the Calliope River in central Queensland. She lost her vision as she struggled to get out of the water.

"It was hurting a lot and I didn't think I was going to make it either," Sam said. "She was saying 'I can't see, I can't breathe'. She's going 'get this thing off me' and said 'am I going to die?'"

Rachel then collapsed on the riverbank unconscious. Quick-thinking nearby campers doused her in vinegar to stop more venom entering her system.

Her father, who does not want to be identified, said it was horrific.

"It was like someone had thrown purple paint all over her legs. Crazy paint, everywhere," he said.

Rachel's mother drove her to meet paramedics near the remote campsite. But her heart stopped on the way and her father had to perform CPR.

"I noticed Rachel wasn't breathing anymore and she was turning blue. That's when I started CPR," he said.

A box jellyfish can kill an adult in less than four minutes.

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Marine biologists are shocked the venomous sea creature was so far from the ocean, with the river 20 kilometres away.