A shark attack on a woman in Southland was probably a case of mistaken identity.

The young woman is recovering from moderate leg injuries after she was attacked while surfing at Porpoise Bay in the Catlins.

Marine biologist Riley Elliot says shark attack statistics are very low in comparison to the number of humans in the sea.

He says it's likely to have been a sevengill shark, that mistook the woman for a seal.

Mr Elliot said sevengills rely on smell and vibration to find food because they have poor eyesight.

"Sharks don't hunt us or eat us, sharks eat fish and other mammalian things like seals."

However, Elliot said surfers need to understand the risk they're taking when they enter the water.

"People should not be naive about the waters they enter, and the ocean is where sharks live. They're very efficient predators and if they wanted to put us on the menu we'd be on there every day."

Elliot said this is because the area has a thriving eco-system that attracts great whites and sevengill sharks.

"Wherever you have healthy ecosystems you have sharks, with healthy and maintained fish populations."