A plan to use old oilfield equipment for an artificial reef near Ningaloo Reef in Western Australia could have endangered the health of people who ate fish caught there, according to a federal government assessment.

WA recreational fishing lobby group Recfishwest wanted to sink an 83-metre-long floating steel structure from a shuttered Woodside oilfield 1.7 kilometres from Ningaloo World Heritage Area to be the centrepiece of an artificial reef but later abandoned the plan.

However, the structure, a riser turret mooring, most likely contained dangerous amounts of toxic fire retardant chemicals called PDBEs.

Ningaloo has the world’s largest concentrations of whale sharks, when up to 500 gather as coral spawns. Aside from the more than 700 species of fish, the coast is home to about 10,000 marine turtle nests every year.

The plan was expected to “present a material risk to the marine ecosystems” and “risks to human health cannot be ruled out”, according to Department of Agriculture, Water and the Environment documents obtained with a freedom of information request by the Australian Conservation Foundation.

If the reef was used for recreational fishing as intended, “it would be difficult to exclude potential risks to humans through ingestion,” a January 2021 assessment by DAWE said.

“PentaBDE is known to have associations with loss of IQ.”

Woodside had planned to tow the structure from its Enfield oil field off the north-west coast to shore when production ended in 2018. However, offshore safety regulator NOPSEMA found Woodside had skipped on maintenance and the ballast tanks needed to allow the structure to float horizontally for a long tow no longer worked.

The alternative method of disposal was to tow the 2452-tonne structure a much shorter distance and sink it in deep water where together with 48 smaller purpose-made structures it would form an artificial reef to create a habitat for fish.