It’s supposed to be high-quality Australian beef, produced from grass-fed cattle raised on idyllic green pastures on the outskirts of Sydney.

But farmers are reeling after learning they and their animals are contaminated with high levels of toxic chemicals from firefighting foam used for decades at the Department of Defence’s nearby RAAF Base in Richmond.

A 9News investigation has exposed the dirty secret brewing in groundwater, soil, wildlife, plants and livestock - which can in turn enter the food chain and end up on our plates.

The high quality Australian beef Alastair McLaren is trying to rear is contaminated by run-off from the nearby Richmond RAAF base. (9news)

These long-lasting chemicals - known as per- and poly-fluoroalkyl substances (PFAS) - are linked to health problems like raised cholesterol, impaired immunity, reproductive and developmental issues, and some cancers.

Blood tests reveal less than a mouthful of meat from the cow with the highest level of PFAS would put a 70kg consumer over the US daily minimal risk level - or a quarter of a medium steak (50g) would put them at the higher Australian maximum Tolerable Daily Intake (TDI)

Farmer Alastair McLaren, who stopped selling pasture-raised, grass-fed beef from his family business Paddock To Plate after they were advised to limit their own consumption late last year, was devastated by the results of blood tests on his family and cattle.

"It's really heartbreaking," he says. " We were immensely proud of the product we were producing. I feel so bad for our customers."

Cattle farmer Alastair McLaren's family and his stock have high levels of PFAS chemicals in their bodies from run-off at the nearby Richmond RAAF base. (9news)

Mr McLaren's own blood and that of his wife Kellie and three of their four children are significantly higher in PFAS than normal. Their eldest son wasn't tested but the whole family regularly ate their own beef.

The family are particularly upset because they only moved their cattle onto land opposite the RAAF base four years ago. Defence has been aware of the PFAS issue since at least 2003.

"We've got nothing left to lose," says Mr McLaren. "We've lost everything."

Blood tests of his cattle reveal levels of the chemicals as much as eight times the proposed trigger point for investigation for PFOS (a form of PFAS) in meat by Food Standards Australia and New Zealand.

Typical blood levels of PFOS are 3.9ng/mL for an Australian adult and 2.9ng/mL for a child.

One of the McLarens' cows, which would have ended up as ground beef, had 390ng/mL of PFOS in its blood. Even a day-old calf had 72ng/mL.

Of the family, Alastair had the highest at 66ng/mL and his wife Kellie had the lowest at 21ng/mL. Their teenage son had 44ng/mL, their 10-year-old son had 39ng/mL and their daughter recorded 23ng/mL.

The high levels of toxic chemicals are from firefighting foam used for decades at the Richmond RAAF base, that runs into the farm's water supply. (9news)

They could choose to keep selling their meat as there are no Australian nor international restrictions on the sale of animal products affected by PFAS.

PFAS chemicals are problematic because they persist in the environment for decades rather than breaking down. They also accumulate in the human body, with a typical half life of between five and ten years.

The situation is complicated because the polluter is the Federal Department of Defence - and the RAAF Base Richmond is technically on federal land.

The state regulator is the Environment Protection Authority (EPA) of NSW, which is aware of the PFAS contamination but cannot regulate Defence.

Class actions have already been brought against Defence over PFAS contamination from RAAF bases by residents of Williamtown in NSW, Oakey in Queensland and Tindal in the Northern Territory.