New testing has found the first New South Wales cases of a super bug that has killed thousands of patients in European and North American hospitals.

New South Wales Health says 21 patients tested positive to a virulent strain of Clostridium difficile, not detected in Australia until earlier this year in Melbourne.

Some of the symptoms include mild to severe diarrhoea, fever and stomach pains.

Professor Lyn Gilbert, chair of the New South Wales expert advisory group on health care associated infections, says New South Wales Health are now testing regularly for the bug.

"Some cases have been identified almost by chance in northern Sydney that have occurred over the last couple of years in several different hospitals in northern Sydney," she said.

"None of them as far as we can tell have particularly severe disease, contrary to what happened in the northern hemisphere where it often was more severe than usual."