From Saturday, punters using the UnionPay system -- around half of those visiting the city from the mainland -- will only be able to get 5,000 patacas (around $600) from ATMs every day, the South China Morning Post reported.
The news sent casino stocks plunging, with Sands China down 4.93 per cent in Hong Kong, Wynn Macau 5.82 per cent lower and Galaxy Entertainment also losing 4.25 per cent by the midday break.
UnionPay International said in a statement to AFP that its overseas cash withdrawal policies "remain the same" -- 10,000 yuan per day with an annual cap of 100,000 yuan (USD 14,500) -- but added it complies with regulations and laws issued by relevant authorities.
The move, if it happens, appears to be the latest attempt by Chinese authorities to stem a growing tide of capital flowing out of China as locals seek safer investments abroad.
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Capital flight is estimated by Bloomberg to have reached USD 1 trillion in 2015 and has continued during 2016, despite recent efforts by Beijing to tighten restrictions on currency flows.
China's foreign exchange reserves, the world's largest hard-currency stockpile, dropped for the fifth-straight month in November to $3.05 trillion, prompting authorities to step up control on capital flight.
Beijing had earlier this week warned against "irrational" overseas acquisitions as more domestic funds were being spent overseas with Chinese company's increasing shopping spree on foreign asset.
It also indicated it could relax restrictions on foreign investment in some sectors.
"Union Pay is the mainland banks' credit card system, equivalent to Visa and Mastercard," Francis Lun of Hong Kong-based GEO Securities told AFP.