In several posts, the most recent of which was added on Thursday morning, Ms Brockovich posted anti-fluoride material and urged her followers to read research claiming that fluoride is dangerous.

"Do your own reading," she urged when posting a link to research claiming that fluoride accumulates in the human pineal gland to very high levels.

Australia's top fluoride expert, Professor Wendell Evans, disputes claims that fluoride damages this gland, while Australian and American health authorities and governments say there is no proof that fluoride is dangerous.

Ms Brockovich came to fame when Julia Roberts played her in the movie Erin Brockovich in 2000. Roberts won an Academy Award for the role.

The film dramatised the single mother's investigation into Pacific Gas & Electric in Hinkley, California, which revealed the company had poisoned the town's water for more than 30 years by leaking toxic chromium 6 into the groundwater. It resulted in a massive settlement for the people of that town.

Since then, Ms Brockovich has been a campaigner for a range of issues, including oil spills and compensation for women who claim to have been suffered side-effects caused by depo-provera birth-control injections.

In Australia, she appeared as an ambassador for Shine Lawyers at a meeting in Oakey, Queensland, to talk about water contamination.

On Thursday morning, Ms Brockovich posted a new update detailing "why the United States' Environmental protection Agency (EPA) Headquarters' Union of Scientists Opposes Fluoridation".The post says that most of its members had thought that fluoride's only effects were beneficial – such as reductions in tooth decay – until recently.

Ms Brockovich's posts have been seized upon by the Fluoride Action Network Australia, which has reposted and shared Ms Brockovich's comments to its own members.

Her comments coincided with a renewed campaign by local anti-fluoride activists ahead of a forum on the topic by US anti-fluoride activist Paul Connett at the University of Technology, Sydney, on February 21.

Dr Connett argues that fluoridation's role in the decline of tooth decay is in doubt, even saying that where fluoridation of the water had been stopped, tooth decay had decreased – an assertion disputed by all public health associations.

Leading health and dental experts have refused to attend the forum, claiming the panel is stacked with those opposed to fluoride and biased.

In Sydney, Ms Brockovich recently told lawyers: "What will be our legacy? Our lasting legacy will be our fight for cleaner water, good land and respect for society."

Ms Brockovich was not available for comment on Thursday.

When asked whether there were any doubts about the efficacy of fluoride, NSW Health said the "overwhelming weight of scientific evidence does not link water fluoridation at optimal levels to any short- or long-term health-related problems, or any environmental impacts".

The NSW government supports the fluoridation of drinking water as an effective, efficient, socially equitable and safe approach to the prevention of tooth decay.

Fluoridation of water is endorsed and supported by the World Health Organisation and the National Health and Medical Research Council as well as leading clinical organisations in Australia, including the Australian Medical Association and the Australian Dental Association.

Studies conducted in Australia also support the efficacy of fluoridation. For example, a recent study in Queensland found significantly higher decay rates in non-fluoridated areas compared with the only long-standing fluoridated community, Townsville.