Hundreds of tar balls have surfaced on Sydney beaches. But what are they?

By Jesse Hyland for the ABC

One of Sydney's most popular beaches came under the spotlight this week when hundreds of black balls mysteriously appeared along its shore.

The sphere-shaped debris washed up at Coogee Beach on Tuesday, resulting in the beach's closure and an investigation by Randwick City Council and the NSW Environment Protection Authority (EPA).

The black balls then surfaced at Clovelly Beach, Gordons Bay and the northern end of Maroubra Beach.

The EPA confirmed on Thursday that balls had also been found at Bondi, Bronte, Tamarama, Little Bay, Malabar, Frenchmans and Congong beaches.

All beaches will remain closed until further investigation has been carried out.

NSW Maritime director Darren Wood said at a press conference this afternoon no more tar balls had been seen drifting towards Sydney's beaches, "which is a good sign at the moment".

"We believe that it is safe enough for you to be out on the water. We don't believe it's safe enough to be at the beach or swimming at the moment, until we can get the full clean-up done," Wood said.

The EPA has confirmed the spheres are tar balls. But what are tar balls? And what caused them to wash ashore?

What are tar balls?

Tar balls originate from oil spills which coagulate into a spherical shape over time.

According to Scientia Professor Matthew England from the UNSW Centre for Marine Science and Innovation, the balls are comprised of oil or other crude hydrocarbon material.

"When oil spills into the ocean, initially, it's not got this structure at all. It's usually a liquid," he said.

"It spreads out over the surface of the ocean, but over time, the oil bonds with other surface pollution, and also surface matter."

Surface matter can include sand or small scale biota, according to Professor England.

"Those tar balls are spherical because of the action of wind, waves and weathering."

Steve Beaman, the executive director regulatory operations at the EPA, said the tar balls along Sydney's beaches had been found to be predominantly oil or hydrocarbon, an organic compound found in fossil fuels like petroleum and coal, based.

"At this stage, we know the material is predominantly hydrocarbon or oil based, and so we think the threat is low," he said.

"But what we want to say to people is the best thing you can do is avoid contact with this material."

Are tar balls dangerous?

If you do contact a tar ball, England said you should be OK if you wash your hands afterwards.

But they can be seriously harmful if they're consumed.

"Nobody should pick them up and eat them, and people wouldn't tend to do that," England said.

"It's like crude oil, it's toxic, it's carcinogenic to ingest, but to touch, you just go wash your hands and you'll be fine."

England noted that the "real danger is through the food chain".

"Humans eating fish, for example, that have ingested this. It's incredibly toxic stuff," he said.

The EPA is investigating the origin and any potential risks to the community and environment.

At this stage, the origin and contents of the balls remains a mystery.

Where did they come from?

While it's currently not clear what the source of these tar balls are, NSW Maritime and the Australian Maritime Safety Authority are currently investigating potential origins.

"[The Australian Maritime Safety Authority] will do reverse modelling and drift modelling, take into account the currents and the wind, for [NSW Maritime] to be able to work out whether it's come from the south, north or east off the coast of New South Wales," Wood said.

England said it was difficult to determine exactly where the tar balls had come from.

"What has confused a lot of the scientists looking at this is that it's such an isolated bit of the coast, that would suggest it's a nearby source," he said.

"But the fact that there are these small tar balls that have formed, that suggests that it's come from a much further distance."

England said the balls could have potentially come from an oil spill "thousands of kilometres away" and spent "at least months, maybe even a year" in the ocean as it formed into a solid structure.

This means there could be more tar balls coming to Sydney beaches.

"That's the worry, is that this could just be a really small bit of a bigger amount of debris that's not quite inundated our coastline," England said.

"You could have this happen if there's a greater amount of material out there."

When will beaches reopen?

Beaches could likely open once the council has cleared the debris on all affected beaches.

But with more tar balls appearing today, that could take a while.

England said it was likely even more would turn up in the coming days.

"I wouldn't be surprised if they do the clean-up and then a few days later we get more of them," he said.

"I think it means we need to be on alert for at least the next couple of weeks about this oil debris, because the chances are there's a lot more out in the ocean that hasn't yet made it to the coast."

While the appearance of tar balls on Australian beaches is rare, England said it highlighted a broader issue.

"It just goes to show our dependence on fossil fuels is causing this problem," he said.

"Extracting oil from beneath the sea floor, and doing this on the ocean, transporting it via vessels, and actually just pumping the oil out, is a risky thing to do. And you have a risk of oil spills, and this is incredibly toxic for ecosystems.

"This is one of the downsides of oil extraction that you get - these tar balls can survive for decades and enter the food chain."

- ABC