As part of the Australian Microplastic Assessment Project, researchers and volunteers sifted through the sand at the high tide mark on more than 300 beaches.

They found more than three million microplastic fragments, with results showing Sydney's Manly Cove among the worst for the tiny particles.

The Sydney beach was surveyed almost monthly for two years, with many readings above 250 pieces per square metre.

One sample in September 2019 uncovered 4051 pieces per square metre, including about 600 pieces smaller than one millimetre.

Other Sydney beaches to record more than 250 pieces per square metre included Watsons Bay, Athol Beach and Botany Bay's Cook Park.

Meanwhile, two parts of Adelaide's West Lakes estuarine system had results of 5500 and 9500 pieces per square metre.

A moderate reading at Warrnambool's Shelly Beach was the worst in Victoria while Fremantle's Bathers Beach North had a moderate amount of plastic detected.

Low or very low amounts of plastic were reported in the handful of samples taken from Bronte in Sydney, Byron Bay and Brisbane's South Stradbroke Island.

Microplastics were usually higher in urban waterways, such as Sydney Harbour and city rivers, with beaches generally lower, depending on which way they face, AUSMAP research director Scott Wilson said.

"One of our general results (nationwide) is wherever we get large population centres, be it cities or a small town, we're finding microplastics," the Macquarie University ecotoxicologist told AAP on Friday.

But regular samples, like those done at Manly Cove, help painted a clearer picture of what's in the water.

"We're always keen to get new people involved," he said.

"We can't be everywhere so we're reliant on those people getting out to their local beaches, lakes and rivers and sampling for us"

The citizen scientists assisting with the research are trained up to use a square frame to scoop the top few centimetres of sand, separate the microplastic from the sand and then sort into pre-defined categories.

The shoreline acts as a surrogate for what's in the water, showing what risks animals and other organisms face in the environment.

Microplastics can come from clothing such as activewear, synthetic grasses, rubbish and other sources. Once in the water, the particles can absorb other contaminants.

"The diesel fuel that comes off the ferry or the metals that wash off our streets will bind to the surface of the plastics, which makes them potentially more harmful to an animal that eats that bit of plastics," Dr Wilson said.

"They're not just eating the plastic - they're potentially getting a whole cocktail of chemicals as well."

Dr Wilson, who next week begins as Earth Watch Institute's chief scientist, said the AUSMAP project has partnered with local governments and state environmental agencies to monitor microplastic levels and track down the source.

The project offered residents and businesses the knowledge to better manage their activities and reduce their deposits, he said.

Australian Associated Press