A giant fatberg, potentially the size of four Sydney buses, within Sydney Water’s Malabar deepwater ocean sewer has been identified as the likely source of the debris balls that washed up on Sydney beaches a year ago.

Sydney Water isn’t sure exactly how big the fatberg is because it can’t easily access where it has accumulated.

Fixing the problem would require shutting down the outfall – which reaches 2.3km offshore – for maintenance and diverting sewage to “cliff face discharge”, which would close Sydney’s beaches “for months”, a secret report obtained by Guardian Australia states.

“The working hypothesis is FOG [fats, oils and grease] accumulation in an inaccessible dead zone between the Malabar bulkhead door and the decline tunnel has potentially led to sloughing events, releasing debris balls,” the report concludes.

“This chamber was not designed for routine maintenance and can only be accessed by taking the DOOF offline and diverting effluent to the cliff face for an extended period (months), which would close Sydney beaches.”