An estimated 800 million bits of rubbish are flowing into Port Phillip Bay annually from just two rivers, painting an alarming picture of pollution on Melbourne’s much-loved coastline.

The figure emerged from a submission to the state government by the Port Phillip EcoCentre environment group, which estimated the amount of mostly plastic litter flooding into the bay and posing a threat to marine life.

Port Phillip Bay keeper Neil Blake and Yarra River Keeper education officer Nicole Kowalczyk conducting a trawl for rubbish in the Yarra River. Credit:Paul Jeffers, The Age

The group has warned that 828 million pieces of litter - of which 74 per cent are microplastics - enter the bay from the Yarra and Maribyrnong rivers every year.

Microplastics are tiny plastic particles smaller than a fingernail, according to the World Economic Forum’s definition. Larger items such as plastic bags are also problematic in Victoria.