Authorities say a large number of South Australian firefighters tested for a toxic chemical have above average levels in their blood

Blood tests on 300 South Australian firefighters have revealed 126 have above average levels of a toxic chemical used in firefighting foam, with 15 found to have extremely high levels.

The Metropolitan Fire Service says most of those in the extreme range work at Adelaide's Largs North station where PFAS contamination has also been found in soil and in a firefighting boat.

MFS assistant chief fire officer Roy Thompson says it is very unlikely further testing will find high concentrations of PFAS at other MFS properties around the state.

"There is no evidence beyond our site of any contamination," he said.

It was originally thought the PFAS chemical exposure had come from food produced in a vegetable garden at Largs North.

The latest tests confirmed higher than normal levels in both fruit grown on site and in the soil.

The PFAS chemicals have been linked to cancer in studies overseas and have been the subject of a recent inquiry into their use on air force bases in Australia.

The Department of Defence recently released details of PFAS contamination at the RAAF base at Edinburgh in Adelaide's north.

The MFS said the firefighting boat at Largs North, the MV Gallantry, would be taken out of service and cleaned.

Further testing will also be conducted at the station on Friday with a meeting of staff also to be held to address their concerns.