Nearly 10 per cent of fish species sold by Sydney retailers are not what they claim to be, a group of high school students has found in the first local study of its kind.

The investigation of two retailers at the Sydney Fish Market and one fish retailer in the Sydney CBD found that two of the mislabelled samples were actually an entirely different family of fish to that listed.

Dr Andrew Mitchell with students Michael Sacks, William Zhan, Michael O'Dea and Patrick Wan, who tested the fish. Credit:Steven Siewert

"Some of the fish, you can't just look at and tell what they are," said Michael Sacks, 20, who was one of 12 Sydney Grammar School students involved in the study, which was conducted in 2015 and 2016.

"I previously had a trust of the label on fish but this has made me realise that's not always the case," said Michael, who is now studying physics and maths at the University of Sydney.