Media playback is unsupported on your device Media caption Divers have captured Sydney's underwater scrapheap on film

Sydney is certainly not usually seen as synonymous with rubbish.

On the contrary - areas around the Opera House, the Botanic Gardens, Darling Harbour and the Central Business District are pristine, spotless.

If you can't quite eat your dinner off Sydney's pavements, you wouldn't need to give them much of a scrub to make them fit for purpose.

No wonder the city attracts nearly 10m visitors a year, more than double its population.

But there is another side to the place Sydneysiders take so much pride in. It lies beneath their feet, or, more accurately, beneath the glistening waters of their beloved harbour.

Under the Harbour Bridge, along the foreshores and below the moorings of the boats docked in front of multi-million dollar condos lies rubbish - lots of it.

Rubbish, garbage, waste, it all clings to the floor of the harbour - bikes, shopping trollies, takeaway food containers and an awful lot of plastic.

'Eco-diver'

Image caption Environmental group Two Hands says rubbish covers the harbour floor

The issue has been brought to light, if not to the surface, by an environmental action group called Two Hands.

They sent down divers to capture the extent of the rubbish strewn across the harbour floor on video.

"It's everywhere," says Dean Cropp, Two Hands' chief underwater cameraman.

Just because this stuff is out of sight under water doesn't mean we can ignore it Dean Cropp, Environmentalist

"There's tons and tons of material which has been washed into the harbour through drains, pipes and tributaries."

Two Hands say the plastic is the most menacing of the pollutants.

"It takes hundreds of years to break down and marine life scoops it up all the time," Mr Cropp says.

The images show plastic bags, containers and crisp wrappers by the thousand, gently rocking to the motion of the water.

The sight would have an almost aesthetic appeal were it not so potent a threat to the submerged eco-system.

"The fish and sea birds ingest the plastic, or they get tangled in it, or they choke on it. Either way it kills them," says Dave Thomas, another of the divers.

A 30-year veteran of underwater exploration, he says: "I'm not an eco-warrior, I'm an eco-diver."

"What all this rubbish is doing is poisoning the very world I spend much of my life in."

He dived in to show us how easy it was to haul up another load of rubbish.

In just 12 minutes under water, he filled two sacks with mostly plastic waste.

"And that's just a few metres from the shore," he says. "If you go a little further out, there's more, gathering in every area of the harbour."

Ocean fears

Two Hands can't put a figure on how much rubbish there is.

But the City Environmental Services Department collects around 900 tonnes of garbage from the various beaches and foreshores in the course of a year. That is just the rubbish that is washed up.

Image caption Divers have found all kinds of objects under water

But there is no systematic programme to scoop up the trash that lies beneath the surface.

"They certainly help with their official clean-ups, but no-one is really tackling the sources of the problem, the people who drop litter on streets far from the harbour, or those who deliberately jettison their rubbish into rivers and streams that feed into the harbour," says Silke Stuckenbrook, another committed campaigner from Two Hands.

The group says the problem goes further than the harbour, literally.

The rubbish is transported by the tides within the estuary, but then it is sucked up into the currents of the Pacific Ocean, which lies beyond the harbour entrance.

"That means it's taken away from Australia, past New Zealand, across thousands of kilometres of open ocean", Dean Cropp says.

Migrating rubbish

The thought of a mass of migrating rubbish spewing out into the world's oceans is one that disturbs environmentalists far beyond the shores of Australia.

In fact one such giant clump of waste has already amassed itself in the Pacific, frustrating oceanographers for years.

It's in the Northern Pacific and even has its own name.

The Great Pacific Garbage Patch, also described as the Pacific Trash Vortex, is what marine scientists call this rotating collection of ocean-borne rubbish.

Image caption Divers were able to fill two sacks of waste in just 12 minutes

The patch has exceptionally high concentrations of plastics, chemical sludge and other debris trapped by the currents.

It's not all fuelled by the products of Sydney Harbour and nor is Sydney alone in having such an underwater problem.

But the city's shame has been exposed by the endeavours of the environmentalists, who believe all leaders should take responsibility for what's happening.

"Just because this stuff is out of sight under water doesn't mean we can ignore it," Mr Cropp says.

To underline his point, Dean shows us one of his most telling under water images.

It's a solitary piece of floating plastic tape, the kind which you see police, ambulance or the fire service put up around accidents to keep the public out.

No doubt it was discarded without malice and with little thought given to its final destination.

But, to Dean, the one word printed on it, serves as an aquatic metaphor for how the world should treat the issue of marine pollution.

The word? CAUTION.