The study, conducted by the University of Adelaide, found microplastic fragments in 49 percent of the fish.
Lead researcher Nina Wootton of the School of Biological Sciences at the University of Adelaide said many commercial and recreational fishermen were unaware of the magnitude of the problem.
“We wanted to know what people knew about microplastic pollution in fish, from fishermen to fishmongers to consumers,” she said in a statement.
“It varied, but about half didn’t know it was a problem because the South Australian oceans had such a reputation for being clean.”
There was three times as much plastic in fish from international waters, Ms Wootton added.
Researchers are now working with the fishing industry to educate fishermen and the community about reducing plastic waste.
“We still have time to ensure that Australian seafood remains among the cleanest in the world, but we need the fishing industry and the general public to recognize the problem and take positive action now,” she said.