Environmental authorities have revealed the cause of mysterious, sticky and stinking grease balls which washed up on Sydney beaches in gross scenes - narrowing them down to one of the city’s own wastewater treatment systems.

The tar-like balls floated onto Sydney beaches between October 2024 and February 2025 and investigations now suggest the fatbergs have reformed inside the tunnel which spits out filtered water into the sea, kilometres offshore.

In a statement posted on Friday, the NSW Environment Protection Authority (EPA) says the Malabar Wastewater Treatment System is the likely source.

The Malabar system runs across 30km of Sydney, starting in the western suburbs.

Authorities say this network is the likely cause because the other wastewater systems pick up less of the industrial

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