However, the federal Department of the Environment concluded the plan presented “a material risk to the marine ecosystems” and “risks to human health cannot be ruled out”, according to documents obtained via a freedom of information request by the Australian Conservation Foundation.

The problem was 65 cubic meters of polyurethane foam in the tower thought to contain up to 120 kilograms of fire retardant chemicals called PBDEs that are associated with loss of IQ. The department concluded they could affect humans who ate fish caught at the planned artificial reef.

Woodside has now reverted to disposing of the tower at the Henderson industrial area south of Perth, but it will now be lifted onto a barge, not towed through the water.

A Woodside spokeswoman said it would act to increase the buoyancy of the tower in the coming weeks, plans to remove the tower in late 2023 and early 2024, and until then, comply with the regulator’s direction to minimise risks to the environment and shipping.

If the tower continues to sink, there is a risk it will end up on the seabed before the planned removal. The Woodside spokeswoman said if that happened the company was committed to retrieving the tower.

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Greenpeace campaigner Richard George said Woodside had made billions exporting Australia’s gas but left a trail of accidents, near misses and shoddy clean-up in its wake.

“If Woodside thinks it’s too expensive or too hard to clean up the mess it’s made in Western Australia’s oceans then it can’t be trusted not to make an even bigger mess drilling for new gas at Scarborough and Browse.”

Australia’s offshore oil and gas industry faces a $55 billion bill by 2050 to remove its equipment from the ocean.

The Enfield oil field where the tower is located is operated by Woodside and is 40 per cent owned by Mitsui, which also runs the Waitsia onshore gas field north of Perth.

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