Despite the price drop, the NCA found little evidence of competition in the market and chief executive Ann Fitzgerald said a new overseas entrant was needed to offer consumers "a real alternative".
The survey found the price difference between Tesco, Dunnes and Superquinn on a basket of 103 items was just 1.14, or 0.4%. A second basket of 87 products from Tesco, Dunnes, Superquinn and SuperValu revealed a price difference of 5.75, or 2.4%.
"There is evidence the most powerful retailers, between them controlling 70% of the market, are still price-matching in core branded items to a significant degree," she said.
Planning curbs on supermarket size should be eased to attract a new entrant to the market and stimulate competition, she added. "Groups such as Asda and Costco might come here if the planning conditions suited them."
Speaking on behalf of the multiples, Retail Ireland said Fitzgerald's comments could not be reconciled with the 14% reduction in prices. RGDATA, which speaks for 4,000 independent grocers, also criticised the survey.