Erin Brockovich-backed plaintiff firm Shine Lawyers is investigating what it says would be Australia's largest class action on behalf of residents throughout Australia whose properties have been contaminated with the toxic firefighting foam PFAS.

National special counsel Joshua Aylward said he estimates up to 40,000 people live in these communities and are affected by these chemicals.

Shine Lawyers ambassador Brockovich, who has been travelling in Australia this week, said the national government has been sending mixed messages to the community.

"Residents are being told it doesn't pose a risk to their health but at the same time the government is giving them bottled water," Brockovich said in a statement. "It's an extraordinarily confusing message."

PFAS, a collection of synthetic compounds, is a fire retardant that was used between the 1970s and 2000 at military defence bases across Australia.

Shine says the chemicals cause "widespread environmental and health concerns" and do no break down in the environment.