New research has revealed the make-up of the mysterious black balls that washed up on Sydney beaches last month.

Scientists from the University of New South Wales have analysed the sticky spheres, initially identified as ‘tar balls’, and discovered that they instead have a stomach-churning human origin.

Researchers found they had more in common with “fatbergs” and contained drugs, so-called forever chemicals ... and human faeces, ABC reports.

“They’re consistent with human-generated waste, the sort of thing that you would have in a sewer,” University of NSW chemistry professor Jon Beves said.

Scientists also found soap scum, cooking oil, fuel, and drugs such as THC and methamphetamine.

Beaches were closed after the mysterious, black, ball-shaped debris washed up. Photo / Randwick Council

He said the “extremely unpleasant” balls were transported around the university as various departments tested them.

“The balls have been sent all over the campus for people to do different sorts of analysis. And yeah, they do smell,” he said.

Despite now knowing what they are, researchers still don’t know where they came from.

“I don’t know if it’s come from our local sewer system, or if it’s been discharged from a boat, or if it’s been washed from the sewers into the stormwater, or some other origin that we really don’t know,” Beves said.

“It’s definitely not from an oil spill. It’s human-generated waste or pollution that somehow made it into the water.”

Thousands of the balls were found washed up across some of Sydney’s most famous beaches such as Bondi, Coogee, and Maroubra.

Despite how widespread they were, scientists believe their consistent composition points to a single source.

The NSW Environment Protection Authority said causes such as wastewater outflow and a spill from shipping are being considered but final testing is due in the coming weeks, which may reveal the source.