He said rice is now selling as low as 1,800 riel per kilogram ($0.45), down from 3,500 to 4,000 riel per kg fetched by private merchants before the order to open the stockpiles came.
Phou Puy added that the association is prepared to release tens of thousands of tons of rice into the market, as needed, to keep prices down.
Sok Savy, 45, a Phnom Penh resident, said she was happy to see the lower prices.
On Wednesday, the government ordered a two-month halt in rice exports and release of stockpiles to try to ease prices.
Prime Minister Hun Sen had blamed the fast-escalating rice prices on "dishonest" merchants and higher oil prices.