Park allegedly received between 50 and 200 million won (now USD 47,000 to USD 188,000) from the National Intelligence Service (NIS) every month from soon after her swearing-in in early 2013 until mid-2016, prosecutors told South Korean media.
The cash -- said to total 3.8 billion won -- was reportedly delivered by NIS agents to Park's aides in uncrowded parking lots or in back alleys near the presidential Blue House.
The money came from the spy agency's "blind budget" -- millions of dollars of special funds that can be spent without receipts for anti-espionage activities -- Yonhap news agency said.
Park is already on trial on 18 charges including bribery, coercion and abuse of power for offering governmental favours to tycoons, and prosecutors will add a 19th count to the indictment regarding the payments, reports said.
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South Korea's first female president was impeached by parliament after months of mass protests over a sprawling graft scandal, and the constitutional court upheld the decision in March last year, dismissing her from office.
She has been in custody since March 31.
Park has since refused to cooperate with a team of state-sponsored defence lawyers appointed by the court.
Prosecutors sought to question Park in prison twice over the fresh allegations, but she has refused to cooperate.
Two former NIS chiefs have been indicted over the bribery charges.