A young girl injured in what was thought to be a shark attack may have been bitten by a mackerel.

A 10-year-old girl injured in what was thought to be a shark attack may have been bitten by a mackerel.

The girl suffered "very minor injuries" to her hand when she was bitten on Tuesday afternoon while swimming off the main beach at Lennox Head, a spokeswoman at Lismore Base hospital told AAP.

Initially it was believed she had been attacked by a shark. But authorities say they aren't sure what is responsible for the injuries.

"They don't know if that's actually correct," the spokeswoman said of the shark attack theory.

Georgia Laddin, from the Lennox Head girls surfriders told News Corp Australia it wasn't a shark.

"It was a mackerel. She's been bitten by a fish," she said.

"She's fine, she's OK. She's one of the local girls who surfs down there."

A police spokeswoman told News Corp Australia the girl's thumb was scratched.

No further detail was available and the girl's family don't want to speak to media.