The project surveyed Australia's coastline at 100-kilometre intervals with the help of 6000 students, teachers and members of the public.

Illawarra beaches had between 10,000 and 20,000 pieces of plastic per square kilometre - the third highest of six brackets in the study.

The worst bracket had 40,000 to 80,000 pieces of plastic.

Manly's Shelly Beach had the dubious honour of having the most plastic debris in NSW.

CSIRO research scientist Britta Denise Hardesty said the findings had a silver lining.

"Australians have typically believed that our litter comes from overseas," she said.

"I think the positive thing about knowing that it is local is that we can enact changes at a local level to reduce or stop litter from getting out into the marine environment.

"Knowing that it comes from us means that we can make changes happen, and that those changes will be effective."

The report investigated the impact local councils had by correlating the amount of debris found and what initiatives local councils had in place.

It found that "councils that had targeted part of their budget specifically to address debris were successful in reducing it".

"Our findings suggest that anti-dumping campaigns, increased awareness campaigns and litter bins on beaches are likely to be effective. I'd also suggest that each of us can have an impact and that with individual behaviour we can make a difference," Dr Hardesty said.

Neither Shellharbour nor Wollongong council have anti-marine debris campaigns but have other initiatives.