Each square kilometre of ocean around Australia is polluted with thousands of 'invisible' fragments of plastic, a new study has found, posing a potential health hazard for humans and marine life alike.

Julia Reisser, a PhD student from the University of Western Australia's Ocean Institute, and colleagues, report their findings today in the journal PLOS ONE.

"This is the first research to quantify marine plastic pollution by directly collecting it from Australian waters," says Reisser.

Plastic debris is well known to be a threat to marine life such as sea birds and turtles that eat plastic or become caught up in it.

But, says Reisser there is a less visible dimension to the story.

"Plastic doesn't biodegrade in water but with the sun and heat it breaks down into smaller and smaller particles over time," she says.

At this small size plastic is also potentially more toxic than the larger fragments because of hazardous materials they contain, as well as pollutants absorbed from surrounding waters.

"Some plastic has plasticisers like BPA that can be toxic but more importantly, when the plastic is out in the ocean, the surface of the plastic acts like a sponge for oily pollutants," says Reisser.

The smaller the plastic fragment, the older the plastic and the longer it has had to collect pollutants such as DDT she says.

"It's a hazard not only for marine vertebrates but also for invertebrates at the base of the food chain," says Reisser.

"I think we need to look into this problem that is less visible but also out there and has a broader impact."

Scooping up plastics

To quantify the amount of these so-called microplastics in Australian waters, Reisser and colleagues surveyed waters between Hobart and Perth, Brisbane and Fiji, and between Auckland and Hobart, Brisbane and Fiji, Brisbane and the Gulf of Carpentaria, and Broome and Ningaloo.

The researchers used a 0.3 millimetre-mesh net to scoop up plastic fragments floating on the surface of the water.

"The majority of the fragments were smaller than 5 millimetres in size," says Reisser.

By knowing the area covered by the net they could then calculate the concentration of plastic fragments.

On average there were around 4000 microplastic fragments per square kilometre although some hotspots had concentrations of around 15,000 to 23,000 fragments per square kilometre.

"As we collected high concentrations of plastic off the cities such as Brisbane, Sydney and Lautoka in Fiji we think that one of the sources is populated areas," says Reisser.

She says while fishing equipment is known to be a source of plastic in the ocean, it is estimated that globally 80 per cent of marine plastic comes from land based sources including plastic cups, plastic water bottles, plastic bags and other plastic packaging.

Some of the hotspots, however, were in very remote areas, such as off south west Tasmania.

"You look at the ocean and it looks really pristine but there was quite a high concentration of plastics there," says Reisser.

She says the concentration was probably due to the convergence of ocean currents in that location.

High concentrations of plastics were also found off the North West Shelf.