The Consumer Council on Tuesday urged consumers to stay vigilant when shopping in supermarkets, noting retailers are taking advantage of downsized products to effect price hikes.

The watchdog said it surveyed 62 supermarket products, including food, daily necessities and pet food with volume or weight changes over the past 3.5 years.

It found that nearly all of them had seen weight or volume reduction ranging from about one percent to as much as 30 percent.

Almost 40 percent of products showed a de facto price increase, with the highest reaching 26.2 percent based on the average per-unit recommended retail price, the council noted.

The council’s chief executive, Gilly Wong said while it is reasonable for retailers to seek profit-making measures, they should also be fair to consumers.

"Because usually for this kind of little change, consumers may not easily spot it except you follow the product for a long, long time. You are so loyal to that, then you follow," she said.

"Otherwise, when there's a modest change, like this time, [a drop of] only three grams of instant noodle or maybe 20 grams of body lotion, it's really hard for consumers to keep track of that."

She noted that while some countries, like South Korea, have implemented legislation to increase transparency regarding changes in product size, such as listing the unit price, this may not be the optimal solution for Hong Kong.

"The reason is Hong Kong is a very open market. We import a lot of products worldwide. We don't have our self-production that much. So, if we ask the government to legislate according to what other markets have been doing right now, we have to consider whether it is the most effective measure," she said.

The council advised consumers to pay close attention to a product's volume or weight, and compare products using unit pricing, if they want to objectively assess their expenses.