The Department of Defence has admitted it should have warned the public about a chemical contamination scandal three years earlier than it did — and says the cost to taxpayers to clean up the problem nationwide will mount into the hundreds of millions of dollars.

Key points: Defence admits it should have gone public about the dangers of perfluorinated chemicals three years before it did

Defence admits it should have gone public about the dangers of perfluorinated chemicals three years before it did The chemicals entered groundwater after being used at 18 bases around Australia

The chemicals entered groundwater after being used at 18 bases around Australia US authorities have warned of a "probable link" between the chemicals and cancer

Defence is now tackling perfluorinated chemical pollution — the source of which was the widespread use of firefighting foam — at no fewer than 18 Defence bases across the country.

At some, local drinking water has been contaminated.

Since at least 2000, scientific research has linked these chemicals to a range of human diseases, though the Federal Government's formal health advice says there is "no consistent evidence" they cause specific illnesses.

Now, a Four Corners investigation has confirmed Defence misused the toxic firefighting foam for decades.

Despite explicit warnings dating back to 1987 that the product must not enter the environment, many thousands of litres of the foam were expelled onto bare earth or washed into stormwater systems.

"There's no doubt about it, that the way we used these products in the firefighting airfields back in the 80s and 90s was not as good as it should have been," department deputy secretary Steve Grzeskowiak said.

Map Defence Force toxic chemical contamination sites

He also agreed Defence had erred in hiding the contamination outbreak for more than three years from the people who live near the Royal Australian Air Force base at Williamtown in NSW.

"I think if we had our time again, should we have told the community back in 2012, from the middle of 2012? We probably should," he told the program.

Defence only revealed the pollution scandal to Williamtown residents in September 2015.

Government stance on class actions a 'slap in the face'

The source of the chemical pollution was the widespread use of firefighting foam. ( CRC CARE )

The admissions will breathe new life into two separate class actions, representing almost 1,000 people, which accuse Defence of negligence and which seek financial compensation.

The Federal Government has so far lodged court papers defending its conduct in one of those cases, denying the claims for financial damages and maintaining Williamtown residents had run out of time to bring the litigation — even though Defence only told them about the contamination two years ago.

Ben Allen, a partner at Dentons, the law firm behind one of the actions, said this position by the Government was "a complete and utter slap in the face".

"They had no way of knowing about the problem prior to September 2015," he said.

Mr Grzeskowiak declined to comment on the court proceedings.

Williamtown residents have been asked to follow precautionary advice to minimise their exposure to PFAS chemicals. ( Four Corners )

No quick or easy solution

The toxic chemicals were critical elements of an aqueous film-forming foam called Light Water which was manufactured by 3M.

In May 2000, the company announced it was phasing out the foam and US authorities warned the Australian Government on the same day that the foam's key chemical ingredients "potentially pose a risk to human health".

Documents show Defence did not fully replace the 3M foam until July 2012.

In just the past year, Defence has spent about $10 million attempting to filter the perfluorinated chemicals (commonly grouped as "per- and poly-fluoroalkyl substances", or PFAS for short) from stormwater drains leaving the Williamtown base and entering the surrounding suburbs.

But testing by Four Corners showed the PFAS chemicals were still at elevated levels three weeks ago in these stormwater canals; at Dawson's Drain, for example, laboratory analysis showed one of the PFAS chemicals flowing from the base at 18 times the safe drinking level.

Dawson's Drain — one of the locations Four Corners tested water for contamination. ( Four Corners )

The environmental scientist who ran a NSW Government inquiry that investigated the Williamtown contamination has told the program there was no way to "feasibly remediate" the site.

"You can treat some of the water in the drains, but there's several of those going off the base, and it's in the groundwater," Professor Mark Taylor said.

"It's just so expensive, it's so complicated … I don't think you really can. It's not a practical option."

Mr Grzeskowiak said the remediation was only just beginning and he couldn't identify when Williamtown could expect to be free of the pollution.

He also said he could not estimate the final cost of the clean-up.

He said that in the past five years, "the figure would be in excess of $100 million either currently expended or committed to be expended".

Asked if the final bill for taxpayers would be "in the hundreds of millions of dollars", he said: "I would say so."

Environmental impacts and 'probable' cancer link

Scientists have established that PFAS does not readily break down in the environment and accumulates in the food chain.

Elevated levels have been found in the blood of residents at Williamtown and Oakey in Queensland, which is home to an army aviation base.

The US Environmental Protection Agency says there is "suggestive evidence of carcinogenic potential" associated with the chemicals.

One major US study found a "probable link" to six diseases including two types of cancer.

There are, however, widely-acknowledged uncertainties in the science.

Experts have urged governments to protect communities now as the potential impact of exposure to the chemicals on human health becomes clearer.

Residents of Williamtown hold the Defence Department responsible for the contamination. ( Four Corners )

Defence has adopted what it describes as a "precautionary approach", supplying bottled drinking water to several communities including Katherine in the Northern Territory, where the town's aquifer has been contaminated as a result of intensive firefighting training on the nearby Tindal RAAF base.

Former Tindal firefighters have described to Four Corners the use of hundreds of litres a week of the toxic foam concentrate in normal training and testing regimes.

This training was so intensive that at Oakey, more than 900 litres of the foam concentrate was discharged every week, according to Defence documents.

Assistant Defence Minister James McGrath said in May the Government was considering compensation options including land acquisitions.

An announcement is yet to be made.

Do you know more about this story? Email besser.linton@abc.net.au

Contamination: The unfolding scandal of toxic water in Australian communities airs at 8:30pm AEST on ABC TV and iview.