Many tonnes of food that could feed thousands of poor and needy is trashed by the four supermarket chains each day.
Green group Friends of the Earth said unsold food dumped by ParknShop, Wellcome, China Resources Vanguard and Jusco amounts to about 87 tonnes - but about one third, or 29 tonnes, of it is edible.
Presenting its waste outrage another way, the group said the amount of food dumped by the four chains' 650 stores each year is equivalent in weight to about 2,000 double-decker buses.
Friends of the Earth conducted a study from January to the end of last month by visiting refuse stations used by five supermarket outlets in Kowloon East, Kowloon West and Hong Kong Island and found an average amount of food waste per outlet of 134 kilograms.
About a third was unspoiled items including bread, vegetables, meat and sushi that had not reached an expiry date. Some meat and bread was good for another week when tossed away.
The refuse sleuths also found that supermarket staff unwrapped some packaged food and poured in water to discourage scavengers.
The alarming study is another wake- up call to authorities who spend their time looking for new landfills rather than reducing waste at source, said Friends of the Earth environmental affairs officer Celia Fung Sze-lai.
Supermarkets not only use landfills at an alarming rate but also deprive the poor of much-needed food, she said.
"Under the extreme disparity between the rich and the poor, grassroots are struggling for every meal. Yet supermarkets dump 29 tonnes of perfectly good food every day. That's irresponsible behavior."
With a market share as high as 53 percent, Fung argued, the chains should show corporate responsibility.
So her group wants supermarkets to donate still-edible food to charity groups and turn expired or rotten food into compost.
Fung also urged officials to follow the example of Britain, which in September scrapped the "best before" food label. That indicates a cutoff date on premium quality rather than a date after which it is unsafe.
"Supermarkets are throwing away perfectly good food with the 'best- before' label because their staff are unaware of what it means and they fear penalties for leaving the food on the shelves," she said.
The four chains each explained about monitoring systems for expiry dates and how food is cleared from shelves in the cause of safety and quality. China Resources also said it will consider donating, food but first it has to study how to ensure quality. And a Wellcome spokeswoman said the chain has held promotions just before the expiry dates of some food products as part of its waste-reduction policy.
Connie Ng Man-yin, service manager of St James' Settlement People's Food Bank, said supermarkets would be encouraged to donate if they were protected should someone fall sick. Donations from supermarkets would be a big step to reduce the reliance of charities on canned food, added Ng, who is involved in a program that helps around 2,000 needy people.
But Friends of the Earth quoted a ParknShop spokesman as saying the amount of food thrown out is minimal and the chain will not consider recycling or donating fresh food to charity.