A total of 17.5 million pieces of plastic waste are flushed from the Shing Mun River into the sea each year, says a Greenpeace study.
In wet weather, 56 pieces of plastic go into the sea from the river every minute, the green group said.
It conducted surveys at nine major rivers in Hong Kong from August - Shing Mun River, Lam Tsuen River, Tai Po River, Tuen Mun River, Shek Sheung River, Sheung Yue River, Yuen Long Nullah, Shan Pui River and Kam Tin River.
It also placed a video camera at Shing Mun River for eight consecutive days to monitor the waste problem.
It found that more than 48,000 pieces of plastic flowed into Tolo Harbour each day. On one day, the amount of plastic waste that flowed to the harbor doubled as there were 88 millimeters of rain and the Strong Wind Signal No 3 was hoisted.
The plastic waste included food packaging, takeaway boxes, straws, tableware, bags, bottles and styrofoam.
Chan Hall-sion, the group's campaigner, said the rivers connect to areas with great ecological value like Deep Bay, Mai Po Nature Reserve and Tolo Harbour.
She said with the waste getting into Deep Bay through the Sheung Yue River, it will also affect Lau Fau Shan as it is along Deep Bay's coastal intertidal zone, where there are rich oyster beds and seafood.
Microplastic levels at mudflats near Deep Bay and Mai Po were two to three times higher than other beaches in Hong Kong, while an oyster sample from Lau Fau Shan was found to have more than 200 pieces of microplastics.
Chan said the plastic waste in the city is one of the main contributors of marine pollution as the waste in the rivers is produced locally.
"One month ago when Super Typhoon Mangkhut hit, a large amount of disposable plastic trash flooded various places in Hong Kong like Heng Fa Chuen, Sai Kung Pier, Discovery Bay and numerous beaches. Fragments of a 20-year-old styrofoam fast-food take-away box were found," she said.
The group estimated that the two biggest food chains in the city - Cafe de Coral and Fairwood - dispensed up to 150 million pieces of single-use plastic utensils last year. It called for the two food chains to establish a plan on stopping the use of plastic immediately.
It said the food chains should create a plastic-free environment, and stop using disposable plastic utensils when people are dining in the restaurants.
They should also gradually reduce the amount of disposable plastics for takeaway items.
The group also suggested that the food chains can offer discounts for customers who use reusable containers for their takeaway meals.