Public funds totalling £104,000 were paid into a fraudulent bank account and are "highly unlikely" to be recovered, a Welsh assembly committee has heard.
The money was paid over a four-month period by the Assembly Commission, which is the corporate body for the National Assembly for Wales.
Its public accounts committee was told police are investigating and an arrest had been made.
The incident is thought to be an isolated one.
Commission member Peter Black told committee members on Tuesday that £71,000 was paid into the fraudulent account in during the 2013-14 financial year, and the remainder was paid this year.
He said it was "highly unlikely" the commission would recover the money.
Members were told it was an isolated incident and the commission was strengthening controls around the way it records and verifies data supplied by outside bodies.
Claire Clancy, chief executive and clerk of the National Assembly for Wales, said: "The strengthened controls will help to ensure the integrity of our payments system and minimise risks from external sources".
An assembly spokesman said it could not give any further information about the bank account or what the money was intended to pay for while police were investigating.