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A secret report reveals the likely source of the fatberg balls that closed a raft of Sydney beaches last summer – with most coming from the deepwater ocean outfalls at Malabar, Bondi and North Head.

The partly-redacted scientific report, obtained by Guardian Australia under freedom of information laws, points to a combination of heavy rains and a buildup of fats, oils and greases as the likely cause of the “poo balls”.

Authorities are unable to say when balls could wash up again – but are urgently working on solutions.

“We do believe that fat is accumulating somewhere in the system. We know [it’s] in the pipes, definitely, [but] we’re unsure of its whereabouts specifically,” Sydney Water’s environment manager, Ben Armstrong, told Guardian Australia.

The report notes that the debris balls “are assumed to be of a sewage origin” and their “discharge is most likely to be from an outfall, but the stormwater system may also be implicated”.

“Fats, oils and greases (FOGs) in the sewerage system provide the ‘stickiness’ for the debris balls. They possibly adhere to the sides of pipes. Particulate material may then stick to these FOGs,” the oceanographic modelling report by WQ Data states.

“High rainfall in the year or so leading up to the discharge of the debris balls effectively ‘primes’ the sewerage (or stormwater) system. Within a few days prior to their discharge, further rainfall is sufficient to move the combined FOGs and particles out of the pipes.”

Wave action then rolls the combined FOGs and particulate matter into balls. Between October 2024 and February 2025, “the oceanographic / meteorological conditions were suitable for the debris balls to be released and for them to be moved to the Sydney and NSW south coast beaches”.

Work by Sydney Water and the NSW Environment Protection Authority (EPA) suggests the so-called fatbergs are due to an increased use of vegetable oils, which break down slowly, an increase in food outlets and the overall growth of the city’s population.

The EPA has previously said testing revealed the balls were consistent with human-generated waste such as grease and faecal matter.

There are particular concerns about the Malabar sewage treatment plant.

“There are global issues about fatbergs and sewers, and it’s effectively the same thing,” the chair of the EPA’s advisory panel, and an independent expert, Prof Stuart Khan says.

“This particular sewer [at Malabar] has had a long time for fats, oils and grease to build up.

“There’s nearly 2 million customers in that system and over the past 10 years, not only has there been growth in population using that system, but also increasing numbers of food outlets that may or may not have appropriate trade waste licenses in place or the right infrastructure in terms of grease traps and things to be preventing run off of fats, oils and grease.”

The release of the oceanographic report, commissioned by Sydney Water, suggests the state-owned corporation could have known as early as 3 February 2025 that the debris balls were likely from its ocean outfalls. That is the date of the “preliminary draft”.

Guardian Australia first reported in October 2024 that a team of scientists was investigating whether the balls could be linked to sewage and whether they could have come from water treatment plants.

The EPA chose to reveal the content – but not the source – of the beach balls on the day of the US election in November.

The EPA publicly confirmed their likely origin in April 2025 when it issued a preliminary investigation notice to Sydney Water.

Since then, the two agencies have been conducting urgent studies to ascertain the extent of the problem.

Sydney’s sewage system relies on three “deepwater ocean outfall” pipes that run up to 4km out to sea at Malabar, Bondi and North Head. The sewage receives primary treatment to remove some solids and is then pumped through pipes, exiting through diffusers 60 metres below the surface, where the plume mixes with seawater.

Treatment plants map

Prior to 1990, Sydney’s sewage was discharged from the bottom of the cliffs near the plants, just a few hundred metres out to sea. Pollution was a regular phenomenon, particularly after rain, with faecal contamination and other rubbish regularly washing onto beaches.

The ocean outfalls were highly successful in improving water quality, with faecal contamination becoming almost negligible, except immediately after rain, when stormwater runoff sometimes caused an issue.

But the growth in population, food outlets and industry, particularly in Sydney’s west, is causing a buildup of fats, oils and grease in the system, which threatens to overwhelm it.

The oceanographic study

The study commissioned by Sydney Water, which mapped oceanographic currents and winds, traced the balls back to more than one of the three major sewage outfalls.

It hypothesises that heavy rain about six months prior to the discharge of the balls, which occurred between October 2024 and January 2025, primed the system and further heavy rain then dislodged the fatbergs.

“No single discharge from a single origin could be responsible for the appearance of the debris ball on all beaches between October 2024 and February 2025.

“Ten (or perhaps more) discharge events may be responsible for the observed debris balls on the beaches during this period.”

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The report studied 10 incidents. The first balls were found on Coogee beach in October 2024. Subsequent incidents occurred at other eastern suburbs beaches and Botany Bay, Manly, the northern beaches and on the south coast.

The report and subsequent studies raise the question of whether Sydney Water’s sewage system has reached its limits and whether debris balls will return to the beaches this summer if rainfall conditions are repeated.

“We’ve had rainfall events and pulses going through the system without having debris balls previously, to our knowledge,” Khan says.

“So we’re trying to find out: why now? And why only sometimes now? Why not after every wet weather event? There are questions around whether the catchment to the sewer is changing, and therefore whether or not it’s something that we’re going to see more often. If that’s the case, then better controls over what goes into the sewer will be important.”

There are concerns about what is lurking in the deepwater outfall pipes.

Sydney Water can inspect other parts of the system, but is unable to inspect these pipes, which are 35 years old. To undertake maintenance would require decommissioning the outfalls and reverting to releasing barely treated sewage at Sydney’s cliffs.

The solution

Armstrong says Sydney Water’s “Save our sinks” campaign encourages businesses and households to avoid putting oils and fats, milk, coffee grounds and other solids down the sink.

Changing consumer behaviour is an important part of the solution, he says. Maintenance is also crucial.

Julian Thompson, operations manager at the Environmental Protection Authority, says Sydney Water has “a fairly stringent” maintenance program, particularly around the Malabar catchment.

“They spend quite a lot of money on desilting their sewers. They have crews that can get into those pipes. It sounds like a pretty unpleasant job, but they’re basically within those large pipes … taking out fats, oils and greases, taking out wet wipes and other things that block up sewers.”

The longer-term solutions require greater investment and some hard decisions for the NSW government.

“What we’re currently doing now is not sustainable – discharging 80% of the sewage produced by 5 million people into the ocean after only primary treatment,” Khan says. “Every drop of water we send out into the Pacific Ocean is wasted water.”

Sydney Water is looking to expand or build water resource recovery facilities which take sewage, highly treat it and render it suitable for reuse. It has a demonstration plant at Quakers Hill.

This could reduce pressure on the existing wastewater system while potentially providing Sydney with a non-rainfall-dependent form of additional drinking water.

A map showing proposed new water treatment sites and Sydney Water’s existing plants, which have deepwater outfall pipes that reach up to 4km off the coast.

“The NSW government made a very clear statement that we would only proceed with that if it could be demonstrated that there’s a social license for it,” Khan says.

Sydney Water’s long-term plan has these projects listed with a 10-year horizon. They may need to be brought forward.

The state’s water minister, Rose Jackson, says: “Sydney Water is now working to prevent future events through new programs to help reduce the amount of fats, oils and grease entering the wastewater system.”

She noted the long-term plan to upgrade the system and reduce the volume of water flowing through the outfalls.

“This approach delivers environmental benefits while avoiding the significant cost to customers of upgrading coastal plants and pipelines to manage increasing flows, she said.