NEW YORK, March 26 (Reuters) - The Federal Reserve's Treasury bill buying is likely to slow notably next month, as planned, the official responsible for implementing monetary policy at the Federal Reserve Bank of New York said on Thursday.

"An adjustment to our monthly purchase pace is likely to happen soon," said Roberto Perli, manager of the System Open Market Account.

While it is hard to say exactly what market liquidity needs will be as the financial system navigates the upcoming tax-payment date, what is now around $40 billion per month in Treasury bill buying "can likely be significantly reduced after April 15," Perli said, adding, "To account for uncertainty and other factors, that reduction may be somewhat gradual."

Perli was referring to large-scale purchases of Treasury bills the Fed embarked upon at the close of last year. The Fed said then it would buy the short-term government debt to rebuild liquidity after having just halted what had been a long-running drawdown of its balance sheet.

This so-called quantitative tightening, or QT, had been running since 2022 and aimed to remove excessive liquidity from the financial system. That allowed the Fed to reduce overall holdings from around $9 trillion to under $7 trillion.

QT ended when liquidity tightened enough to threaten the Fed's control of its benchmark interest rate.

To ensure orderly conditions, the Fed announced its bill buying, which is also aimed at shortening the overall maturity of Fed bond holdings to better match the Treasury market.

In his remarks, Perli said the size of the buying was in part aimed at getting ahead of the tax date issue.

The interest-rate-setting Federal Open Market Committee "could have waited until the expected flows materialized in April to fill the reserves gap, but this would have implied the need to purchase very large amounts of Treasuries in short order," Perli said. "That would have been impractical from an operational standpoint."

He added it's possible the Fed may add more liquidity than needed and that may temporarily depress money-market rates and drive some cash to the Fed's reverse-repo facility, where financial firms can park excess cash overnight at the central bank.

Perli also said in his remarks that eligible financial firms should use the Fed's standing repo operations to borrow cash when it makes sense for them.